Big Name Fan: Akira Kurosawa was such a big fan of his friend Ishiro Honda's work on the Godzilla series he wanted to direct his own.
Executive Meddling: Ishiro Honda, the director of the first film and many other Toho kaiju movies, preferred science fiction/alien invasion movies (like The Mysterians or Gorath) to kaiju efforts, but the monster movies were so successful that he didn't really have a choice. This is not necessarily a bad thing, as he still made excellent movies, but Honda's non-kaiju movies are often unfairly overlooked. This would explain why some of his entries had space monsters or alien invaders or both for Godzilla to fight. One of his movies even takes place on Planet X.
Also, Shusuke Kaneko originally wanted to use Anguirus and Varan in a Godzilla movie he was directing. However, the executives at Toho Studios told him he had to use two more popular monster. The end result? Godzilla, Mothra, King Ghidorah: Giant Monsters All-Out Attack.
Akira Kurosawa, one of the most respected filmmakers of the 20th century, wanted to direct a Godzilla movie, but Toho didn't want to spend the money (as Akira was known to go overbudget and himself cost a lot to direct a film in adittion to the Special Effects cost).
Name's the Same: Raymond Burr's character in the American recut of the original is named Steve Martin. For his scenes in Roger Corman's cut of Godzilla 1985, they deliberately avoided using the character's full name.
Also, Baragon and Barugon (from the second Gamera movie, Gamera vs. Barugon).
Miss Namikawa in Invasion of Astro-Monster and Defense Commander Namikawa in Godzilla: Final Wars (both played by Kumi Mizuno), though the latter was an intentional nod/tribute to the former.
General Segawa in Terror Of Mechagodzilla, and Defense Director Segawa in the Heisei Godzilla films (both played by Kenji Sahara).
Goro Maki the reporter character from Son of Godzilla and Goro Maki the reporter character from The Return of Godzilla.
Trope Namers: Godzilla was the source of the following trope names:
What Could Have Been: Godzilla was originally going to be a giant octopus, then later a giant fire-breathing gorilla, before Toho decided to make him the radioactive dinosaur we all know and love today.
Likewise, Godzilla himself was originally going to be portrayed via stop motion animation. The idea was quickly dropped when the filmmakers realized it would be too time-consuming.
King Kong vs. Godzilla eventually came out of Willis O'Brien's desire to make a King Kong vs. Frankenstein.
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.
Frankenstein Conquers The World was originally going to be Frankenstein vs. Godzilla. The idea was dropped when Toho realized what a one-sided match up it was, coupled by the fact that by the time they had thought up the movie, Godzilla had turned from villain to hero (and Frankenstein would've obviously been the hero of the story). The movie wasn't entirely scrapped, however: Toho simply replaced Godzilla with Baragon.
Toho was going to loan Godzilla out to Tsuburaya Productions for the studio's 10th anniversary. The resulting movie was going to be called Godzilla vs. Redmoon, in which two monsters — Redmoon and Erabus — appear in Japan. The military brings them together in the hopes that they will fight and destroy each other, but it turns out the beasts are male and female of the same species, and they mate! They hatch a little monster named Hafun, whom is unfortunately killed by an entrepreneur trying to capture him. When Redmoon and Erabus predictably go berserk, Godzilla appears and engages the two in battle. The movie was all set to be made when it was abandoned for reasons unknown. It would eventually be heavily reworked into a stand-alone movie without Godzilla, called Daigoro vs. Goliath.
Henry G. Saperstein, the American producer who helped with Monster Zero, Frankenstein Conquers The World and War Of The Gargantuas, liked the Gargantuas so much that he suggested a Godzilla vs. Gargantua movie. Unfortunately, the idea never got passed the planning stage, and it's unknown if Godzilla was going to face Gaira, Sanda, both or a new Gargantua entirely.
When Toho was trying to bring Godzilla back during the late '70s, they decided to cash in on the supernatural/Satanic craze that most horror movies were into at the time (like The Omen and The Exorcist) with Godzilla vs. the Devil, a joint production between Toho and UPA Productions. Mankind's sins begin to take physical form, creating monsters that resemble a spider, a fish and a bird. Godzilla arrives and defeats the demons, but Satan appears to battle Godzilla himself. One can only imagine how a showdown between the King of the Monsters and the Prince of Darkness would've turned out.
One of the more interesting ideas was Godzilla Vs the Asuka Fortress. The concept for this production was penned by longtime series writer Shinichi Sekizawa along with producer and then Toho president Tomoyuki Tanaka. Despite what the title might suggest, Godzilla's rival in this production was to be a humanoid robot constructed by the Self Defense Force that was either part of the "Asuka Fortress" or it was to be the title character itself. Details are unfortunately very scarce (the only well known thing being that the film's plot dealt with a government revolt), and among those things uncertain is what type of role Godzilla was to play. Due to the revolt within the government, which was certainly an interesting story vehicle on the part of writer Sekizawa, it's quite possible that the "Asuka Fortress" was going to be manned by a rogue part of the government and the movie could have seen Godzilla slip into his hero persona to defeat it. In fact, it's quite possible that this movie would have continued the Showa series, although given the lack of details it's hard to say for sure either.
A concept proposed by writer Ryuzo Nakanishi in June of 1978, titled King of the Monsters: Rebirth of Godzilla, with the first portion of the title in English and the second in Japanese, the production was to be a remake of the original 1954 Godzilla film. The concept struck a cord with producer Tomoyuki Tanaka, as a more complete draft was commissioned. This second stage of production had both Nakanishi and writer Akira Murao at work on developing the concept, and was submitted on October 22nd, 1978. Jun Fukuda was set to direct the movie, but for whatever reason the project never came to pass. Given the very late draft submission, it's also worth noting that this film was possibly going to be slated for a 1979 release instead, unless the project itself ran behind schedule early on and was axed for that reason.
Right after Godzilla vs. Biollante, Toho decided to bring back their second biggest star, Mothra, in her own movie: Mothra vs. Bagan. The two creatures were to engage in a series of battles across the globe before Mothra and the Mothra larva joined forces to defeat Bagan in a fight to the finish. Aside from Super Godzilla, this unused script is probably what Bagan is best known for.
Would you believe that the film Gunhed started life as a Godzilla project?
Also planned was a '90s remake of King Kong vs. Godzilla, for Toho's 60th birthday. The idea was dropped when Toho discovered that rights to Kong had become ridiculously expensive since 1962.
Even sooner than that, There were plans for a direct sequel back in the 60's. Following the immense success of the first film (which when adjusted for inflation would be the highest grossing Godzilla film of all time), it's not surprising that the first concept considered for the next Godzilla movie was one that pitted the two monsters against each other again. Literally titled Continuation: King Kong vs. Godzilla, the proposal for this project was written by Shinichi Sekizawa, who also worked on the previous film, and the draft was submitted in 1963. Unfortunately, the concept never got past the proposal stage, and a more fleshed out script was never commissioned while Sekizawa's ideas for the sequel remain mostly unknown.
Apparently, a few monsters were cut from the final version of Final Wars; Megaguirus, Kiryu, Oodako, Destroyah, Ganimes, and Megalon to be precise. Megaguirus was used as stock footage, Kiryu was left out due to budget cuts and was replaced by the Gotengo, Destroyah, Megalon, and Oodako were dropped, and Ebirah replaced Ganimes.
This is not true. Only five monsters were considered for "Final Wars" that didn't make the final cut: Kiryu, Oodako, Gorosaurus, Mothra Larva, and King Ghidorah.
Kiryu, Gorosaurus, and King Ghidorah, were all replaced by Gotengo, Zilla, and Kaiser Ghidorah respectively.
Daiei actually approached Toho on making a Godzilla vs Gamera movie. Which Toho rejected. That sound you hear is your fandom crying.
As said, Famed director, Akira Kurosawa once expressed a desire to direct a Godzilla movie, but was turned down by the good people at Toho, who were terrified by the thought of the budget that might be required to realize Kurosawa's vision.
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.
In 1994, an attempt to make a different American Godzilla movie was made:
It was to have Industrial Light and Magic provide effects and have Godzilla fight an alien creature called The Gryphon. ILM ended up dropping out. But even with Stan Winston offering his services (and even designing both monsters), the movie was never made due to Sony freaking out at the $120 million budget.
The game Godzilla Unleashed has a couple of rejected monsters:
King Kong, and Zilla - The two american kaiju were hinted to be in the game in an interview. However they never made to the final cut of the game; King Kong due to rights issues with Universal and Zilla due to lack of popularity.
Fire Lion, The Visitor, and Lightning Bug - The other conceptualized "new" monsters to be voted into the game. However they all lost to Krystalak and Obsidius.
Mechani Kong - Same reason as Kong.
Gamera - Rights issues with Daiei.
Bagan - Dropped in favor of Varan due to Simon Strange liking Varan moreso.(Guy never gets a break huh folks?)
Word of Saint Paul: Haruo Nakajima, the original actor who played Godzilla, believes that the Showa (1955-1975) incarnation of Godzilla is female. However, according to Toho Studios, all incarnations of Godzilla to date have been male (yes, even the remake version).