The main reason that the first Godzilla movie was made in the first place was because Tohos original big movie of 1954 In the Shadow of Glory had to be cancelled because of political tensions between Japan and Indonesia. The idea to make a monster movie in its stead was because producer Tomyuki Tanaka was daydreaming about a giant sea monster while looking out the window on the return flight.
During conceptualization of Godzilla (1954), Godzilla was originally planned to be a giant octopus, and then a fire-breathing ape (obviously based on King Kong) with a head like a mushroom cloud, and a cross between a whale and a gorilla (hence the name Gojira, a combination of the Japanese words for gorilla and whale) before the producers watched The Beast from 20,000 Fathoms and decided to make Godzilla a prehistoric reptile like the Rhedosaurus, but keeping the name.
Godzilla was originally going to be portrayed with stop-motion similar to King Kong and the Rhedosaurus, but this was largely scrapped (only one scene where Godzilla knocks down a building with his tail was stop-motion) in favour of the iconic monster suits due to the amount of time and budget stop-motion would have required (it was projected that had they shot the film with entirely stop-motion, it would have taken an additional seven years to complete).
Godzilla was initially going to be a monster motivated by hunger, and when his head first appeared over the ridge of Odo Island in his initial appearance, there would have been a cow in his mouth. This was scrapped partly because of logistics (the cow would have realistically been too small to see in Godzilla's mouth from that distance) and partly due to it being perceived as too graphic for the film.
This version of Godzilla was also conceived of as having massive ears. Both of these details would later be recycled for Baragon in Frankenstein Conquers the World.
The original version of Dr. Yamane (Gojira) was going to be more of a caped corny villain who wanted Godzilla to leave. Even going so far as to sabotage operations against him. Thankfully all but his desire to have Godzilla live, to study for the good of mankind, were removed over time.
Godzilla Raids Again was released in the US in an edited form as Gigantis: The Fire Monster. However, the original idea for the movie's US distribution was even more radical. Godzilla Raids Again would've been "remade" into a completely different film known as The Volcano Monsters by AB-PT Pictures. Godzilla and Anguirus would've been referred to as simply a giant T-rex and Ankylosaurus, the monsters would've attacked San Francisco instead of Osaka, and every scene with a Japanese actor would've been cut (along with everything else that the later Gigantis dub cut as well). Toho even lent their suits to AB-PT, since they were desperately trying to market their films to the US in any way they can at the time (even if it involved massive editing). Before the project could come to fruition, however, AB-PT Pictures closed shop. The result? The Volcano Monsters (along with several other films they were developing) never saw a release, the two monster suits that Toho lent went missing, and what we got instead was the Gigantis: The Fire Monster release by Warner Bros..
Anguirus was originally going to have a split carapace similar to a beetle, something that would again come up in conceptual designs for the Heisei versions of Anguirus which never ended up seeing the light of day.
Godzilla Raids Again was originally planned to have an incredibly bizarre immediate sequel in 1956 called Bride of Godzilla, which would center around a giant Robot Girl created by a scientist to combat a third Godzilla and second Anguirus, only Godzilla to fall in love with her (yes, really!). It also would also take place in a Hollow Earth world discovered by the characters the characters and home to a whole ecosystem of Godzillas, Anguiruses, and other monsters (like a giant chameleon and a mutant archaeopteryx). At the end the robot would have detonated its nuclear heart, killing Godzilla and destroying the ecosystem.
Did we mention the giant robot girl would have been naked?
Rodan was originally conceptualized as a giant prehistoric bird resembling Archaeopteryx (whether this had anything to do with the scrapped Bride of Godzilla's Archaeopteryx is unknown) before he became a giant pterosaur instead.
Moguera from The Mysterians was originally going to be an organic kaiju, a quadrupedal monster resembling a cross between a mole and a lizard that could shoot gas from its tail, but it was changed into a (bipedal) mech to better differentiate it from prior Toho monsters (but still retaining many of the design aspects of the original idea).
King Kong vs. Godzilla was originally conceptualized as a film where Kong would fight a giant ogre-like monster created by Dr. Frankenstein (sometimes known as "Prometheus"), before the script was shipped to Toho, where Frankenstein was replaced with Godzilla, and we got one of the most legendary crossover-versus movies of all time, while Frankenstein would later be used in Frankenstein Conquers the World.
Toho initially wanted to shoot on location in Sri Lanka, but since the rights to Kong slashed the budget, they resorted to shooting on their own Oshima Island.
Due to its enormous box office success, a direct sequel called Continuation: King Kong vs. Godzilla was announced but never evolved past the proposal stage. The script for the film involved Kong briefly fighting a giant scorpion, as well as revealing that Godzilla was actually going to be killed at the end of the previous film, only to be brought Back from the Dead here.
Frankenstein vs. Godzilla was also considered but it was scrapped in favour of Mothra vs. Godzilla since the reasoning for Godzilla and Frankenstein encountering one another didn't make sense (the JDSF would awaken Godzilla to fight Frankenstein... even though Frankenstein hadn't done anything destructive and Godzilla hadn't evolved into the heroic phase of his character yet).
Manda was originally going to be a "mammoth snake" but since the year that his film was going to come out was the year of the dragon, they made him into a dragon instead.
Mothra vs. Godzilla was drastically different than what we got with the movie released in 1964. Godzilla was intended to have a bigger role. He was intended to have washed ashore and be a tourist attraction instead of Mothra's Egg, and upon awakening, was to attack the Country of Rolisica, instead of Nagoya. There were no Mothra Larva at all in the movie, only the Imago Mothra, who would've fought with Godzilla at the film's climax and won.
Originally King Ghidorah was supposed to be crimson before he was designed as golden, and some photos of Ghidorah, the Three-Headed Monster in production exist where he is blue with rainbow-coloured wings, which is also the version on the poster.
Mothra's imago form was originally supposed to have appeared along with the larval form, but she was dropped during production over concerns about the effort it would take to have to puppet three flying monsters together, since Rodan and King Ghidorah already took a lot of strings to operate.
BatmanVs. Godzilla. Supposedly, some of the concepts from the film were recycled in Son of Godzilla like the weather control machine. It's probably worth noting the idea came around not too long after the famous 60s Batman show debuted. The closest this ever came to fruition was likely the Toho film Latitude Zero (which is usually considered part of the Godzilla canon-verse), which starred Cesar Romero (who played the Joker in the Batman show) as the villain, who had an army of mutant animals, which included Bat Men (and yes, that is their official name).
You remember Godzilla vs. The Sea Monster? King Kong was originally going to be the star monster instead of Godzilla. This is also evident by most of Kong's mannerisms being recycled into Godzilla's personality in the film, namely Godzilla's odd fascination with Dayo. Kong was replaced by Godzilla because it was intended to be a film adaptation of The King Kong Show, and Rankin/Bass Productions felt the story was too different from the show, leading to the creation of King Kong Escapes instead. But rather than scrapping the story entirely it was retooled to have Godzilla instead of Kong.
Destroy All Monsters. Beyond the intention of being the last Godzilla movie, many other monsters were intended to appear but never made the final cut like Gaira the Green Gargantua, Maguma the giant walrus from Gorath, and Ebirah. Baragon and Varan were also intended to have larger roles, but the former suit had been damaged badly from usage in Ultraman and the latter suit hadn't been used in years, so it was replaced with a motionless prop for a few shots. Amusingly, the dub refers Gorosaurus as Baragon because the scene of Paris' destruction by a tunnelling Gorosaurus originally called for Baragon (or more accurately, it originally called for Maguma, who got replaced by Baragon, who got replaced by Gorosaurus).
King Kong was also slated to appear, but Toho ended up losing his licensing rights early into production.
An early draft of the film was called All Monsters Attack Directive (not to be confused with the later Godzilla film All Monsters Attack), which not only included the appearance of every single Toho kaiju that had appeared prior, but also supposedly would have featured hybrid monsters created by crossbreeding with one another. However, most of this script was scrapped due to budget constraints.
Ishiro Honda initially wanted all of the monster footage in All Monsters Attack to be brand-new, and he also wanted to include a sequence with Godzilla fighting Rodan and Oodako (the Giant Octopus) and Rodan stalking Ichiro attempting to eat him. However due to scheduling and budget issues, the crew was forced to use stock footage for all monster scenes that weren't Godzilla, Minya, and Gabara. As for the Rodan and Oodako fights, they roles were "recycled" for the Kamacuras and Ebirah fights respectively.
Speaking of All Monsters Attack, Gabara started as a giant mole cricket monster known as Gebara. This monster eventually was reused and became Megalon.
Space Amoeba was originally going to be more of an epic with a global scope, involving an alien invasion resulting in the sinking of entire continents, and nuclear weapons used in retaliation, before the story was considerably downgraded to take place on a single (fictional) island. It was then supposed to film on-location in Guam, but even more budget constraints meant that most filming was done on the Japanese island of Hachijō-jima (in winter), while the water scenes were shot in pools in Tokyo.
Eiji Tsuburaya was also supposed to be the special effects supervisor, but he died only two days into production and is uncredited in the final film (a decision made by Toho executives which enraged many of the staff who worked on the film, especially Tsuburaya's pupil Sadamasa Arikawa, who basically disowned the film).
A proposed sequel to Godzilla vs. Hedorah, by the director Yoshimitsu Banno, had Godzilla fighting a giant mutant starfish, which he scrapped in favour of a direct sequel where Godzilla would fight another Hedorah, but in Africa this time. However, this too never saw fruition, reportedly because the Yoshimitsu was sacked before production could begin on the sequel.
Godzilla vs. Redmoon would have been a film by a collaboration of Toho with Tsuburaya Productions (of Ultra Series fame), in which Godzilla fights a family of monsters. It was scrapped for unknown reasons, and Tsuburaya eventually transformed the film into the very bizarre Daigoro vs. Goliath, although it ultimately has very little similarity to the original story involving Godzilla.
Godzilla vs. Gigan originally had two different scripts that were eventually heavily rewritten (largely for budget-related reasons):
The first draft was focused heavily on King Ghidorah and had the evil dragon team up with Gigan and a Red Dragon known as "Mogu". They were going to fight the team of Godzilla, Rodan, and Varan. In this draft, Gigan had a ball-and-chain for one hand.
The second draft was closer to what the film wound up becoming introduced the Space Hunter Nebula-M Aliens and the concept of Gigan teaming up with Ghidorah, as well as Godzilla and Anguirus as a tag-team. The only differences was that Megalon was also introduced to team up with Gigan and Ghidorah and a deity named "Majin-Tuol" teaming up with Godzilla and Anguirus.
There's also now a recently discovered third draft called Godzilla vs. Gigan: The Return of King Ghidorah. This one tried to bring Megalon back in, along with Larva Mothra, but both were scrapped.
Godzilla vs. Megalon was originally to be a solo venture for Jet Jaguar (some believe as a Pilot Movie for a TV series which never ended up happening). However, Toho threw in Godzilla and Gigan at the last moment in hopes of boosting its ticket sales.
Godzilla vs. Mechagodzilla: An earlier draft for the film had Godzilla, Anguirus, and Mothra teaming up to fight aliens called the Garugans. Mothra was replaced with the new King Caesar and the Garugans became the Black Hole Aliens and Mechagodzilla.
Originally in Terror of Mechagodzilla, Titanosaurus was going to be two plesiosaur-like dinosaurs, a male and a female, known as Titans, which were normally peaceful, but became aggressive after entwining their necks.
Toho almost made a film starring a mutated Loch Ness Monster called Nessie, which was going to be a Japanese-British co-production with the legendary horror film studio Hammer. It got as far as a full script, posters, storyboards, and props being made before it was scrapped, since Hammer's golden age was coming to an end and by the time filming was to begin, they were in no shape to start and eventually Toho lost interest after several delays.
It is believed that Henry G. Saperstein had approached Toho about making a movie with Godzilla fighting one or more Gargantuas. Sadly, the concept went nowhere, although Godzilla did end up fighting the Gargantuas in the comic series Godzilla: Rulers of Earth.
Another film collaboration with Saperstein which never ended up materializing was an animated movie featuring Godzilla that was slated for a 1988 release. An animated format was chosen because it would have been much cheaper than a live-action film featuring the same story, whatever it was.
A VERY psychedelic story known as "A Space Godzilla" (and no, not that SpaceGodzilla) was written in 1979 by Katsuhiro Otomo (a.k.a. - The writer and director of AKIRA!) and was even proposed to Toho as a possible film (allegedly to be done with Stop Motion!). It details about a female alien that resembles Godzilla dying of diabetes and its Alien Son (which also resembles Godzilla) traversing the universe, reuniting with its Alien Godzilla father, and ultimately becoming one with the Cosmos, all while being told in a style similar to2001: A Space Odyssey. It also involved the Alien Godzilla father fighting a large-breasted alien that threw swastika-shaped knives.
An arguably even weirder attempt to revive the Godzilla franchise was the proposed God's Godzilla which had Jesus Christ as an evil space alien who animates Godzilla from a hidden, dinosaur-shaped Nazca line drawing in order to send humanity into an apocalypse in which the only survivors are horrific mutants.
Like King Kong (1976) Toho had seriously considered remaking the original film in the Seventies with the title King of Monsters: Rebirth of Godzilla. Apparently however, production got hit with delays before it could even start, leading to its cancellation.
His concept first appeared in an early version of The Return of Godzilla, which would have him fight Godzilla as THREE monsters that could shape-shift individually and would merge together at the film's climax.
The scrapped Mothra vs. Bagan movie gave Bagan his recognizable appearance and the plot of it sounded EXTREMELY similar to that of Rebirth of Mothra. In this scrapped idea, Bagan could FLY.
Godzilla vs. Bagan, a film that was to based on the video game, Super Godzilla. It was going to feature Gotengo and even Godzilla turning into SUPER GODZILLA!
On a side note, the basic design for Super Godzilla was recycled as Spacegodzilla.
Biollante's rose form in Godzilla vs. Biollante was at one point replaced by a fish-rat creature named Deutalios, whom Godzilla was going to kill and then eat. This was cut due to its graphic nature (the second time that a scene showing Godzilla eating meat was cut). Biollante was also originally planned to have only one form and a human face.
The box office disappointment of Godzilla vs Biollante is primarily what led to Toho to go back to using older, popular monsters for most of the Heisei Era instead of creating new ones, such as following up with King Ghidorah, and then Mothra, and then Mechagodzilla. It also killed the conceptualization of a Bagan film that had been planned to follow the film had it been successful.
A remake of King Kong vs. Godzilla was planned for the Heisei Godzilla series (to coincide with Toho's 60th anniversary) but didn't happen because Turner Entertainment wouldn't let Toho use Kong without demanding expensive royalty payments. It supposedly involved Kong being turned into a cyborg before fighting Godzilla. Reportedly, Anguirus had also been considered to be in it as well. Whatever the case, the film eventually evolved into Godzilla vs. King Ghidorah.
After Vs King Ghidorah was a hit, Toho wanted to do an immediate follow-up with another King Ghidorah that would have been of extraterrestrial origin like his Showa era counterpart. According to Koichi Kawakita the project didn't go forward because executives felt it was too easy to bring Ghidorah back again, as well as them seeing a poll at the time showing Mothra was popular among women.
A movie featuring Mechani-Kong fighting Godzilla while a team of scientists get injected into Godzilla's body also didn't happen due to Mechani-Kong's "striking resemblance." It is said that the mecha Bulgario featured in the toku series The Justirisers was based on the unused Heisei design for Mechani-Kong.
An early draft of what would become Godzilla vs. Mothra, Godzilla vs. Gigamoth (involving Mothra, as well as a mutated Mothra known as Gigamoth, the concept of which probably evolved into Battra), sound INSANELY interesting.
There was a proposed concept for a direct sequel which had Mothra's body be turned into a robotic cyborg known as MechaMothra.
Rodan's role in the film was originally going to be given to Titanosaurus.
There were also going to be two Rodan similar to Rodan's debut film, a mated pair who think Godzilla Junior is their son (rather than Rodan thinking he's his brother in the final film). The male would be killed early on, while the female would be defeated and resurrected as "White Rodan" (rather than Fire Rodan) later on and fight Mechagodzilla to her death defending Godzilla Junior (this version is what ended being illustrated on the poster).
Some early ideas for Mechagodzilla was that it would have an artificial alien life-form known as Berserk which would gradually get larger and more Godzilla-like by consuming machinery, and being comprised of seven separate vehicles which would combine to form Mechagodzilla (similar to White Rodan, this unused version is what was illustrated in the film's poster).
Moguera's role in Godzilla Vs Spacegodzilla was originally intended for Mechagodzilla (rebuilt after the event of Godzilla vs. Mechagodzilla II. However, the producers came to the realization that the film had too many Godzillas (Godzilla, Little Godzilla, Spacegodzilla...), so Mechagodzilla was switched out in favour of a more obscure Toho giant robot.
Originally Spacegodzilla was going to be called Astrogodzilla, had telepathic powers, and commanded an army of alien dragonflies. Mothra was also going to have a much larger role where she teamed up with Moguera and Godzilla. Most of this got cut for budget concerns.
Even before this, Toho planned on having Godzilla fight a reimagining of the Showa King Ghidorah known as Emperor Ghidorah, which was an extraterrestrial that was actually capable of manipulating gravity with his gravity beams rather than just shooting lightning. However, he was deemed too similar to the Orochi from the Toho film Yamato Takeru, which was coming out at about the same time, and scrapped, but retaining the idea of Godzilla's opponent being from space, Spacegodzilla was born.
Speaking of Yamato Takeru, had a franchise spawned from it Toho had plans to include Godzilla and King Ghidorah in one of the sequels.
Oh not only that! Toho actually had two monsters already planned for the Yamato Takeru sequel; one was the revived Orochi. The other? Mr. Down on his luck Monster himself: BAGAN!
After production wrapped on Vs Spacegodzilla Toho's writing staff had a contest to decide what draft should be the next Godzilla movie. One of these was a direct sequel titled Spacegodzilla's Counterattack.
There were supposedly plans for a King Ghidorah solo film in the 1990s, but it was scrapped in favour of him featuring as an antagonist in Rebirth of Mothra.
One idea for a Godzilla film in the 1990s was to have Godzilla fight a ghost version of the 1954 Godzilla. The idea was to have the '54 Godzilla win and was also slated to feature Godzilla Junior and Anguirus (concept art was even commissioned to try and design a "Heisei series version" of the character◊), and also involved Anguirus fusing with Ghost Godzilla and Baragon. However, since Godzilla already fought against two Godzilla-like monsters (Mechagodzilla and Spacegodzilla) Toho decided to use the Oxygen Destroyer concept instead to bring the franchise full circle, and thus Godzilla vs. Destoroyah was created. Anguirus (along with a slimmer variant known as the Anguirus Hound) was reportedly scrapped because the four-legged walking stance of the suit made it difficult to portray a realistic sense of speed.
Worth noting that the image on the left in the commission art link isn't Anguirus but rather a different monster called "Barubaroi". Barubaroi was yet another monster made to fight Godzilla in his own film, and would later evolve into Destoroyah (Anguirus again was apparently set to appear as well as Junior, and Anguirus would have been assimilated by Barubaroi).
Another version developed in the conceptualization of Godzilla vs Destoroyah was Godzilla vs. Giant Monster Varan, and as the name implies, the final enemy of the Heisei series was going to be Varan, as the "herald of the apocalypse". Unlike the other proposals however, Godzilla was supposed to live at the end of the film, with the ending being him and Little Godzilla (who finally learns to breathes fire through the course of the plot) swimming off into the sunset.
An early script of Godzilla vs Destoroyah involved a mecha known as G-END constructed from the remnants of Moguera from the previous film, Godzilla Vs Spacegodzilla, but Toho decided against it since it would have meant three movies in a row featuring mechs, which they thought was excessive.
Another idea made during the conceptualization of Godzilla vs Barubaroi (which became Godzilla vs Destoroyah) was that Godzilla would fight a genetically engineered cybernetic humanoid with a human face and skin made of Godzilla cells known as the "Biomonster", and the two monsters would perform a Mutual Kill. There were many other ideas as well, including one where Godzilla fought his son, one where he fought "deep-sea life" (a kaiju resembling Titanosaurus), one where he fought "Super Atomic Godzilla" in Russia, and a particularly wacky one where Junior was sent into the past to battle the Godzilla of the 50s.
Originally, Destoroyah was going to fire a powerful "stomach beam" from the floral pattern on his chest, something seen in concept art, kept in the manga adaptation (where it sliced off the end of Godzilla's tail and one of his dorsal plates), and filmed (where it was strong enough to knock Godzilla over with one shot), but scrapped in post-production.
Destoroyah's death was also longer and more violent, as he would have survived the JSDF's ice tanks, but is then grabbed by Godzilla as he starts melting down, and rendered flightless by his shredded wings, Godzilla would have taken his son's killer to the grave with him, the extreme heat from the meltdown along with the ice tanks still blasting at the two monsters causes Destoroyah to dissolve into nothing. This ending scene was scrapped because it took the focus away from Godzilla's death, although the manga adaptation kept it.
Millenium Era Films
Tanaka's first attempt to revive the Godzilla series after Destoroyah was a proposal titled Godzilla Vs Bio Godzilla, in which a now adult Junior would team up with the Ghost Godzilla to fight off the titular threat. Tanaka's passing ended up leaving the proposal on the cutting room floor.
During production of Godzilla 2000 there was consideration to have King Caesar and Anguirus appear either as opponents or allies, but it was quickly discarded. Possibly due to how Toho doesn't have confidence in their drawing power.
An early draft of the film conceived the film as another solo Godzilla outing much like the first Showa and Heisei films. The story was thrown out for being too vague however, and the alien invasion subplot was added after more drafts.
Concept art for designs of Orga varied extremely widely. Some designs were much more strongly Godzilla-like, others were more bio-mechanical in appearance to the point of not even having a mouth, while one design strongly resembles Legion.
In the early 2000s, film studio Kadokawa-Daiei Pictures approached Toho with the proposal of a Gamera vs. Godzilla crossover film. In what might be one of the most unfortunate missed opportunities in monster film history, Toho rejected the offer, and Kadokawa-Daiei went on to make Gamera the Brave instead. note Gamera and Godzilla did crossover once or twice (depending on what you count as a "crossover) although never in a film, once in a live stage show at the Osaka World Fair for ten days in 1970 (two images of Godzilla and Gorosaurus alongside Gyaos survive, but apparently no pictures of Gamera), and second in the video game City Shrouded In Shadow, although the two never encounter one another or interact in the game (although characters from several different franchises appear, none of their scenes overlap).
Additionally, Kaneko's other ideas for a Godzilla film included him fighting a mutated astronaut known as "M" and a separate one where he fights Kamacuras as Kaneko wanted to experiment with a completely CGI version of the giant mantis.
Remember Kamoebas' corpse appearing in Godzilla Tokyo SOS? Well, that was originally going to be Anguirus! However, the producers did not like the idea of Godzilla's closest ally appearing dead from an offscreen battle with Godzilla, so the next idea was a Liopleurodon, which also got cut because Toho felt it would be useless to introduce a new monster and have it only appear as a corpse. Kamoebas was used instead to placate Anguirus fans and deliver a stealth Take That! towards Gamera.
Anguirus was also initially considered in the prior film, Godzilla Against Mechagodzilla, where he would have fought against Kiryu similar to his encounter with the Showa Mechagodzilla.
After Tokyo SOS wrapped production Masaaki Tezuka submitted a proposal for a solo Kiryu spin off film. It never came to pass.
Godzilla: Final Wars had a number of scrapped elements and kaiju, such as Gorosaurus (replaced by Zilla), Kiryu (replaced by the Gotengo), the Giant Octopus from King Kong vs Godzilla, and King Ghidorah (replaced by Monster X and Keizer Ghidorah).
Concept designs for Godzilla also looked radically different to any incarnation of the character, and looked like snakes with arms and legs than anything else. Another scrapped design looked like the KiryuGoji suit, but with red eyes, a Biollante-like chest pattern and a spiky skin texture, and one model design looked so radically different from the normal Godzilla design that it was hard to even see it as Godzilla.
Additionally, Anguirus, Rodan and King Shisa/King Caesar were supposed to be killed, but the fact that the scene would require them to destroy the suits (which were going to be used for publicity shots and promotions), as well as the fact all three were allies of Godzilla's in the 60s and 70s, caused them to scrap it.
Also at its early stages, Final Wars was to be a Distant Finale to the Heisei films. With the Godzilla being the adult Junior and Destoroyah being his ultimate opponent at the end.
An American Godzilla movie was planned as far back as 1983 and would have been in 3-D. The plot would have been similar to Gorgo as Godzilla would destroy San Francisco in search of his son, not knowing that his son was killed by a Soviet submarine and the corpse was undergoing scientific analysis. Godzila would have also have been killed at the end. His design was also drastically more dinosaurian, with a Tyrannosaurus-like head. Because no studio wanted to spend millions on a "kiddie picture", the film was scrapped along with a potential Rodan remake, although Toho really liked the concept.
The film was originally going to have Godzilla fight against a giant monster named the Gryphon and have special effects done by none other than Stan Winston Studios (some models of the Gryphon, and its minions called Probe Bats were even constructed). The concept of the Gryphon has some What Could Have Beens of its own:
A lie that has been circulating was that the reason the Gryphon never manifested was because Toho veto'd the idea, and proposed Mothra and King Ghidorah instead. Both were turned down because their rights were extremely expensive. The real reason was actually worse.
The real season is that Sonys executives disagreed about the budget and caused the would be director Jan De Bont to drop out. There were several attempts to re-negotiate, and get a director, Tri-Star brought in Roland Emmerich and Dean Devlin. The condition they agreed to direct was that they would rewrite the entire thing however they wanted, and we all know how that went. The even sadder irony is that the budget went way over the budget Sony didnt agree on, meaning that the whole screw up was really meaningless.
The Gryphon started out as a monster with the features of a cougar, cow and bat; and Godzilla's origins would be that he was a bioweapon made by an ancient civilization to fight off the Gryphon. Gryphon ended up becoming an alien creature that would have used the DNA of cougars, horses and bats (as well as snakes for its tongues) to construct a body for itself, killed the script's main villain and ultimately was killed by Godzilla after giving him hell in battle. Compare this script, Godzilla (1994), to the final film, Godzilla (1998), which was far, far less cool.
The film was going to be much closer in feel to the Toho versions, but it clashed too harshly with the director's imagining of the lizard in a more realistic light (for instance, Godzilla looked and moved more like his Toho counterpart, but he looked too much like a man in a rubber suit and broke the suspension of disbelief the director was going for). Some concept art exists of this Godzilla using atomic breath, but in the film proper he did not because of this.
Briefly in conceptualization, the film executives asked for a possible sidekick character for Godzilla (so that they could have a protagonist monster that they would always have the rights to and possibly spin a spin-off franchise from). Understandably, both the director at the time and Toho shot the idea down quickly.
Additionally, the film was originally going to be given a sequel as well. It was primarily set in Australia, and involved Nick raising and nurturing the Baby Godzilla (from the ending) into being in adult, who in turn lays a litter of his own babies. All while trying to balance out how to protect Godzilla and his babies from the world and vice versa, a swarm of insectoid monsters lead by a matriarch known as the Queen Bitch (yes, that is the real name) begin attacking. The concept of Nick raising Baby Godzilla became the basis for the animated series following the movie, which was MUCH better received due to being closer to the original films.
There was an American-made sequel to Godzilla 2000 planned known as Godzilla Reborn, where Godzilla would arrive in Honolulu for an unknown reason, the military would knock him out (in the original script he was actually going to be killed, and then cloned later on), and then a bat-like lava kaiju known as Miba (said to sort of resemble the winged MUTO but without legs) would have erupted from Mauna Loa, causing the characters to realize this is why Godzilla arrived in Hawaii. They wake him up and a Behemoth Battle ending commences. It was scrapped after Columbia Pictures wasn't interested in a film with such a low budget (a proposed twenty million dollars), and Toho would have required other studios to pay a high cost to use the script. Planned actors that might have starred in the film included Bruce Campbell, Christopher Lee, Leonard Nimoy, and Jamie Lee Curtis among others. Perhaps coincidentally, the idea of Godzilla arriving in Honolulu to fight a winged kaiju in an American-made film would be reused thirteen years later.
The project started off in 2004, in the months before the release of Godzilla: Final Wars, as an IMAX 3D short film remake of Godzilla vs. Hedorah tentatively called Godzilla 3D to the Max that even had Yoshimitsu Banno as director at the time where he would have fought a Hedorah-like monster called Deathla. Over the course of 2007-2009, the project eventually morphed into a feature-length film under the aegis of Legendary Pictures.
The trailer with Oppenheimer speaking featured a few scenes of destruction cut and showed a huge, multi-armed woodlouse kaiju, apparently dead. It's not seen in the movie. Fans commonly refer to this monster as "Vishnu", due to Robert Oppenheimer's famous Destroyer of Worlds speech playing in the background, but in the concept art book Godzilla: Art of Destruction it's only referred to as "a strange, dead monster".
The "Art of Destruction" book included concept art of a dogfight between the Male MUTO and some fighter jets.
It also included an idea of the original script which was that the Male MUTO was presumably killed during the Hawaii fight but was actually cocooning and growing wings.
The MUTOs were originally known as Hokmuto (the male, because he was supposed to emerge in Hokkaido) and Femuto (the female, because she's the female) and their EMP abilities were supposed to nullify Godzilla's atomic breath, although that only ended up making it into the novelization.
Godzilla was also originally conceptualized as being six-hundred feet tall before being downscaled to three-hundred and fifty-five feet in the final film, which was still considerably taller than he had ever been before that.
The cinematographers originally planned for the scene with Joe Brody's interrogation and outburst at the MONARCH facility to be shot using several elaborate camera angles and cuts. Bryan Cranston did such a good job with the first rehearsal take that they decided to just use that one and scrap their previous plans.
There were apparently several attempts to keep Joe Brody alive in the script.
Very early into development, the MUTOs were known as Rokmutul and Pterodactyl, which had more strongly reptilian designs, but these later became the female and male MUTOs respectively and ended up having more insectoid designs.
In The Stinger of Kong: Skull Island, Godzilla was planned to have had a physical cameo, with several of the characters on a boat in the Arctic encountering him breaking through the ice. However, this clashed with the perceived timeline established in the previous film which implied Godzilla never surfaced anytime between 1954 and 2014, so instead Godzilla only appears as cave art alongside Mothra, Rodan, and King Ghidorah.
There were also plans for Godzilla to appear in photographs early in the film, but the director did not want a movie about Kong to show Godzilla first and instead the photographs were replaced by images of a wrecked ship strongly implied to have been attacked by Godzilla.
An earlier reference to King Ghidorah was also planned when Marlow exposits the story of Kong and the Skullcrawlers to the other characters, with the wall paintings around them showing multiple types of monsters, including one with "three heads", but in the end only Kong and the Skullcrawlers were shown.
Concept illustrations and statuettes indicate that portraying Rodan with bird-like feathered wings was considered before the final design of the more classic pterosaur-like wings was used (although with the addition of a molten texture to go along with his volcanic origins this time). Other concept art gave him a more batwing-like design.
Behemoth was originally going to be more elephantine, before a more gorilla/ground sloth-like design was used, and Methuselah was conceptualized as being more strongly bull/rhino-like before its more mountainous design.
Some concept illustrations from the art book had unused Titan designs, including several which resembled Toho kaiju such as Gigan, Kumonga (even if a giant spider-like kaiju did end up in the final film), Anguirus (even if his skeleton did end up in the film in a freeze-frame cameo), Kamacuras, Baragon, and even one which looked like Gamera (even if most of these were probably just meant to be placeholders).
There was also concept artwork of Titans which did not appear in the final film, including a scorpion monster with rock-like pincers, an octopus/cuttlefish-like monster, and an aquatic monster called Margygr which appeared as a corpse.
Rodan's introductory scene was originally planned to be even longer than it already was. When Rodan broke free from the volcano, numerous harpoon turrets would have fired to keep him grounded, and then Monarch would have willingly let Rodan loose to fight the approaching Ghidorah. Another sequence was cut involving a fighter jet soaring low over Rodan's back, firing missiles at him as it's doing so, and more shots of fleeing civilians.
It was originally intended by the showrunners to be darker in tone and more faithful to the films than what it ended up being (supposedly, it was originally intended to follow the events of Godzilla Raids Again), but the network executives and television standards at the time would only allow a light-hearted, family-friendly program (Godzilla wasn't even allowed to destroy buildings or breathe fire at people).
Toho wouldn't give the rights to any other monsters besides Godzilla, and even then, the producers weren't even allowed to use Godzilla's roar. Instead, Hanna-Barbara had to make do with creating standard Monster of the Week foes for Godzilla (which, to be fair, were nothing new for Hanna-Barbara), and hired Ted Cassidy to provide Godzilla's generic roaring sounds.
Gunhed was originally written as a Godzilla movie! More specifically, it was originally one of the scripts submitted as part of a contest to create a Godzilla film (the winner of which became Godzilla vs. Biollante), but rather than being scrapped, this script was heavily retooled.
A sequel to the NES game Godzilla: Monster Of Monsters that focused on Rodan was apparently planned for release in 1991, but was reportedly scrapped and replaced with Godzilla 2: War of the Monsters because Toho didn't think Rodan's name had the same selling power.
Kumonga was supposed to appear in Seattle during the events of Godzilla 2000, but was cut due to it being seen as too repetitive after the Kamacuras arc.
The fifth novel, Godzilla and the Lost Continent, was announced but cancelled when the 1998 film bombed; it would feature the Earth itself (personified as a returned Biollante and an original stone/fire monster named Raijin, or an existing kaiju had Toho not agreed) striking back against mankind while undoing much of the environmental damage that mankind had done over the years. The titular "lost continent" would also be occupied by returning monsters Varan, Manda and Battra, the survivors of a long-lost civilization, and a "monstrous forest growth", creatures resembling the Triffids, which would appear around the world.
The sixth novel, had it gone beyond drafting stages, would have included The Mysterians in its continuity, with the title characters having invaded Earth but been repelled (as they were in the film), but returning in the present day with their Moguera to beg for help from Earth.
Cerasini also had plans for Titanosaurus and the Gargantuas (who would have had a non-Frankenstein based origin).
In what can be considered a very tragically missed opportunity as of 1998, famed Japanese director Akira Kurosawa was a big fan of the Godzilla franchise, and wanted to direct his own installment of the series. Unfortunately, Toho turned him down, fearing the enormous budget that his epic style of filmmaking would require.
In the 1998 interactive video game Godzilla Movie Studio Tour (which is less of a game and more of a clip editor with Godzilla films, and including bonus collections of Godzilla-related images and short videos to browse through), there exists mention (in the form of a single concept artwork and multiple images of a model) of a kaiju known as "Dogolas" (or Dogoras), which vaguely resembles a purple Facehugger. It has never been mentioned in any other Godzilla media nor is it even alluded in the game itself to what it is (and in the Japanese version, its name is only mentioned in the game's code), leading to the huge question mark of where it originally came from or was originally intended for.
During conceptualization, there were numerous kaiju created that never ended up in the show, or only appeared in the intro, including many there were named after Toho kaiju, such as Gigan, Gabara, Biollante, Anguirus, and Moguera. However, Word of God has stated that these concepts were never meant to actually represent these monsters, since almost all of them were In Name Only anyway and it would have been too costly and time-consuming to license the rights to all the monsters from Toho.
There was originally planned to be an assortment of technology that would have allowed Nick to ride on and "pilot" Godzilla, which partly ended up being included in a retooled form in the episode "Vision", where Godzilla is outfitted with goggles that allow him to see the Monster of the Week.
The One-Shot Character monster Komodithrax was going to be a female Godzilla before the idea was abandoned in favour of having her a mutated Komodo dragon so that there was still only one surviving Godzilla.
A Trendmasters toy line (which has had a long history of licensing Godzilla toys, including having a short-lived line of merchandise for the 1998 film, the poor sales of which itself ended up having the second intended line to be scrapped) was intended to help promote the series, and several prototypes were even made, but the company went bankrupt shortly before they could be released.
Save The Earth's story mode had an actual plot that was cut during development, with the green barriers being set up by the Vortaak and Vorticia posing as a reporter. The story had to be scrapped because Atari rushed the game so they could release it to coincide with Godzilla: Final Wars. Concept art also exists for an Arctic stage, a Las Vegas stage (showing Godzilla 90's wielding the guitar on the Hard Rock Hotel as a club), and a Rodan rail-shooter taking place in the desert. A handful of the challenges were originally developed for the original story mode, such as Undersea Battle.
Biollante was completed (several other kaiju were considered, including Battra, Super Mechagodzilla, and Titanosaurus, but Biollante was the only one that was actually completed) and to be playable, but was removed from the console versions due to Pipeworks not getting the green light from Toho (In 2014, though, hackers utilizing the PCSX2 emulator (a PlayStation 2 emulator for PC) actually managed to unlock her data, revealing that her entire moveset was recycled for Unleashed).
Hedorah was seriously considered for both Save The Earth and Unleashed, but his complex blob-like body was difficult to model and the inability for game engines at the time to handle the proposed in-game transformation between forms.
There was an IGN fan poll for Unleashed which had the option of four different original kaiju to be added into the game, the Fire Lion, the Visitor, the Lightning Bug, and Magmouth. Magmouth, later renamed Obsidius, ended up being the winning entry.
In an interview numerous other monsters were discussed for Unleashed, but stated by Word of God were only briefly considered early in development before being discarded for various reasons, including King Kong, Zilla, Clover, Gamera, Mechani-Kong, and Monster X.
Some other scenes that were cut included more shots of civilians fleeing and the destructive aftermath, a scene where Godzilla's fourth form was feebly crawling around on all fours (which was also present in the concept art), a scene where he was actually slithering on his tail like a snake, and a scene where he shot his atomic breath at the ground so that it momentarily propelled himself into the air.
Some ideas for Godzilla's design were also that he would have a second head (implicitly that he was reproducing by fission), that the fifth form would emerge from his dorsal plates, and that he would have started off looking normal but gradually became weirder as the film went on, while in the film proper the opposite occurred.
Several concept art designs for Godzilla in Godzilla: Planet of the Monsters depicted him having horns, face fins, ears, and/or fur, with many having a more mammalian-looking face than the final production, which is relatively simplistic.
A single Japanese news article mentions Spacegodzilla being part of the anime trilogy's canon. However, he is ultimately one of the few Toho kaiju that is not even referenced in any way (even the Gargantua/Frankenstein were mentioned to have been considered by the author of the prequel novels), even though many highly obscure monsters that have never interacted with Godzilla before were included (including Daghara, Maguma, Dogora, creatures from Latitude Zero, and even Gamera ended up getting a stealth reference). Whether this was simply a misreport or Spacegodzilla was considered at one time but scrapped is unknown.