One Piece had two prototypes before the chapter that eventually became the first one, and there was heavy rewriting between them. We'll just say that Luffy's character was way toned down and close the discussion. Also, there was no titular treasure (Luffy pretty much became a pirate for the hell of it), and the series was called Romance Dawn (which was used as the name for the manga's first chapter and volume). The first Nami-like character was named Silk, who joins Luffy's crew, along with Galley, according to the Time Skipped epilogue.
In the second Romance Dawn, which was submitted as the pilot, Luffy's grandfather (who's a Marine vice-admiral in the actual manga) played an equivalent role to Shanks, which Eiichiro Oda did so that Shanks would have more of an impact when he was introduced in the actual manga. There's a character named Ann with a similar design to Nami and the position of a swordswoman of limited skill, but she doesn't join Luffy's crew in this version. Plus the Big Bad of this pilot is a pirate captain named Spiel the Hexagon whom to this date is the ONLY One Piece villain who uses magic, such as having the ability to ride on a broomstick, set a subordinate ablaze, summon a hammer, and control water to sink his own ship.
The story of the first prototype was adapted into an animated special made for the JUMP Super Anime Tour that's set in the continuity of the main series between the Thriller Bark and Sabaody Archipelago arcs (following the manga, as the anime hadn't gotten to this point yet). Even before this, the villain named "Crescent Moon-Galley" made a cameo appearance in the anime version of the Loguetown arc.
Captain Kuro makes a reference to the crescent moon that would otherwise be a throwaway line if not for the possible reference to Romance Dawn.
Initially, Oda intended for One Piece to only last for about five years, with the Straw Hats facing only the Four Emperors. Then, Oda devised the concept of the Seven Warlords of the Sea and incorporated them into the story, resulting in the manga lasting far longer than five years (with it taking five times that length to enter its Grand Finale).
Zoro was originally conceived as one of Buggy's bodyguards and the Master Swordsman of Buggy's Pirates, while Cabaji was the knife thrower of the crew instead. Buggy himself looked more like a serious Monster Clown than the goofy Villainous Harlequin he is in the final version of the manga.
Nami was initially going to have an axe as her main weapon. One of her early designs also featured her having scars, as well as a cybernetic-looking left arm and right foot.
At the beginning, Oda planned to make Hatchan a Squid Fishwoman named Marly Reed.
Oda actually did want to include some more stories for the Loguetown arc (though what was intended to be canon is unknown. At the very least the Daddy the Father and Caroline story was meant to be used). But he wanted the crew's journey to start at Chapter 100, hence the rushed pacing in the manga.
Daddy Masterson was originally going to appear in the manga, but Oda nixed the story so that the Straw Hats would leave for the Grand Line in the 100th chapter. Instead, Usopp just buys his goggles off-page and Masterson's story is only seen in the anime, and is thus considered filler by those who do not know this.
Vivi was initially nothing more than an enemy, and her being a princess is the product of pure chance: one time, Oda drew Miss Wednesday with her hair down, which made him think she looked like a princess. Since he had already planned out the plot for the Alabasta arc, this gave him the idea of making her the princess of Alabasta, and the rest is history.
Mr. 2's initial designs looked very different, he was named "Mr. Happy Birthday", and he had a female partner named Miss Obon.
Robin was going to have a technique called Face Fleur, which would have allowed her to duplicate her face. It was rejected almost immediately after it proved too creepy in practice.
Originally, CP9 were going to be much different: Kalifa was a black-haired creepy woman, Jabra was a swordsman called Jakeen, instead of Kumadori there was a Gorilla-themed brute named Apollo, and Fukuro was actually well-muscled instead of being a fatso. Furthermore, the battle layout was different, and included Zoro vs Jakeen, Nami vs Fukuro, and Usopp against Blueno, amongst various things.
Oda mentions that he had intended to have Brook join the Straw Hats before Franky as he always intended for Brook to join the crew, meaning that Thriller Bark would've happened before Water Seven. However, because Kazuki Yao's performances as Jango and Bon Clay were so well liked, he created Franky and had him join before Brook.
Kuma looked very different, and was supposed to have a steering wheel embedded in his head, until the steering wheel was given to Shiki instead.
Nola, the giant snake from the Skypiea arc, was originally intended to be an octopus.
Moriah was initially meant to be a pastor with a split personality, and looked nothing like his final design. Perona also used to be more like a silent princess-type character.
One of Nightmare Luffy's early designs is somewhat reminiscent of Freddy Krueger's.
Brook went through several designs as well; one showed him alive with Round Hippie Shades, another as a more serious-looking skeleton with a bandanna and no afro, and yet another with a top hat.
For the Dressrosa arc, Oda had nearly thirty other names options for Rebecca, like Violet, Girasole, Akiria, Noir, Iris, Isbell, Fabula, Russian, Laura, Helianthus, Seneca, Juvenalis, Vineyard, Lambda, Amazonia and Shalom. Some of those options were considered for meaning "sunflower" in other languages, while other options became the names of other characters, like Violet (Rebecca's aunt's alias) or Russian (Senor Pink's late wife).
Judge's early design looked very different, and he was initially named "Saint-Germain", with his Red Baron being "Count Karakuri".
The Vinsmoke siblings were originally designed to be all boys, each of them sharing a distinct age gap (Ichiji was 30, Niji was 27, Sanji was 21, Yonji was 12). Niji was originally the tallest of the Vinsmoke boys, and Reiju did not exist; instead, Goji would've been the fifth and youngest child at 12 years, and Yonji's twin. Yonji also looked vastly different, being a portly, messy-haired version of Sanji, an appearance that the scrapped Goji character also shared.
The Green Databook reveals more information about the parts of the series that did not make it into the manga:
Among other things, Dadan was going to be a man, Usopp (though with an older design) was going to be the first mate, the shipwright was a short Fishman with a mallet (Franky was there, but he looked completely different and was considered the weaponsmith), and Sanji was originally named "Naruto", but JUMP rejected this as they were still optioning the Naruto pilot to become a series. Well, that explains his eyebrows...
Even more: Moria was originally conceptualized as a kindly-looking priest, while Doflamingo looked like a DJ/rapper, Blackbeard's last name was "Everything", Akainu had glasses and Kizaru was cheerful in a jovial sense instead of the creepy one. Also, Don Krieg had made himself into a mecha and the Amazon Marguerite had a Zoan Devil Fruit called the Cat Cat Fruit: Model Black Cat which made her look like a Cat Girl when transformed.
In a Log Book, a series of sketches were printed, one of which was Chopper with Robin's arms, legs, and eyes on his back. When asked about this, Oda said he was originally intending on Robin to transport the unconscious crew via this method at the end of the Skypiea battles, but he realized this would be such horror for his younger readers that he replaced the idea with something simpler. Still, it brings up an untapped idea of how Robin could easily dispose of enemies without using her basic "Clutch" attack.
According to Oda, in SBS 98, the Animal Kingdom Pirates Card Game Theme Naming is a holdover from an earlier draft of the Wano Arc where Doflamingo, captain of a crew with it's own Card game Theme Naming, originally played the role of a supporting antagonist.
Kappei Yamaguchi had initially auditioned for Luffy before landing his role as Usopp.
Yasunori Matsumoto was originally going to appear as a regular, but scheduling conflicts with Sazae-san ended those plans.
When Toei was shopping around for companies to broadcast One Piece in North America, certain recording studios were asked to produce test episodes for Toei to view. One of the studios in the running to dub One Piece back in 2004 was Funimation, which was already working with Toei on Dragon Ball Z; they were originally considering Eric Vale as Luffy and Christopher Sabat as Helmeppo. However, Funimation was not the only company Toei was considering – they also apparently got a test dub from Blue Water Studios in Calgary, and there were plans for a test dub from Westwood Studios in Vancouver. The Vancouver dub featured Andrew Francis as Luffy, Samuel Vincent as Zoro and Jillian Michaels as Nami. Unfortunately, all of those plans were nixed when Toei sold the license to 4Kids. Funimation would eventually buy the One Piece license in 2007.
Funimation wanted a man to voice Luffy in their dub of the anime as opposed to Colleen Clinkenbeard; Greg Ayres was one of the actors Funimation had in mind. However, Toei and Oda nixed the idea.
Randy Tallman was originally going to play Gan Fall in Funimation's dub. Due to Tallman's November 2007 death, however, he was replaced by John Swasey.
Before the license finally expired, 4Kids did have a few voices selected if they had continued onward to the Skypiea saga for Bellamy, Blackbeard, Eneru and his Priests (dubbed Vassels here) and Wyper and the Shandorians which can be heard in the English version of the Pirates Carnival game. See here.
Films
In Oda's initial draft for One Piece Film: Strong World, the Big Bad Gold Lion Shiki was building a Weather-Control Machine on a sky island which needed four Green Rocks to power it (said Green Rocks were actually referenced in the very first teaser), and this was presumably the genesis for the kidnapping plot. It was also written to be an intense piece, but Oda threw the whole thing out because he felt that it was too forced.
Noé Velázquez, who reprised his role as Sanji from the 4Kids release, was originally selected to direct the dub but declined due to time limitations. Patricia Acevedo accepted because her son is a fan of One Piece. Said son, himself, Rodrigo, would go on to adapt the script for the dub and voice Sabo.
Gabriel Basurto auditioned to reprise his role as Roronoa Zoro from the 4Kids release. However, Dafnis Fernández eventually returned to voice Zoro.
Video Games
There's a lot of non-playable characters in the One Piece: Pirate Warriors games, such as Hatchan, Vista and Ivankov. Some, like Sentomaru are basically clones on existing characters with limited movesets. Others, like Mr. 3, have extensive and unique movesets which suggests that they might have been intended to be playable at some point. However, Ivankov out of the mentioned ones eventually was made playable in the third game, along with others like Magellan and Lucci.
In the leadup to the fourth game, the game's official website advertised Sengoku as one of the characters who was going to be playable for some time before taking his entry down, implying that he was originally going to be playable, but for whatever reason that didn't pan out.
According to longtime fan Randy Troy, Disney approached Oda for the chance to produce the live-action series in 2016, but the studio was turned down in favor of Tomorrow Studios.
Production of the live-action series was set to start at the end of 2020, but the COVID-19 pandemic delayed it.
Benjamin Wadsworth was approached for the part of Luffy. Iñaki Godoy was eventually selected to portray the role.
Madison Kauffman auditioned for the role of Alvida.
Ludi Lin was considered for the role of Zoro.
Nicholas Hamilton was considered for Koby.
In the Latin American Spanish dub, Noé Velázquez was originally considered to voice Sanji again, and he did a few takes for the role. However, the director heard that Taz Skylar himself was native in Spanish and opted to have him dub himself instead.
The first season was originally planned to be ten episodes, but was eventually reduced to eight episodes.
Dean Damonse was cast as Monkey D. Dragon; however, the only scene he appears in is during Gold Roger's execution as the man in the green cloak (which also means that the character is never actually seen in the flesh). On Damonse's Instagram, he says his scenes were cut from the final release.
Chouchou was going to have a bigger role and have his own story with Zoro and it was going to be different than it was in the manga, but it was too different and they ultimately scrapped the idea because they felt that it's important for Zoro to be with Luffy and Nami when dealing with Buggy instead of being on a separate side story.
Hachi was originally planned to appear and he was going to fight Zoro, but he was cut due to budgetary concerns.
In an interview he gave to Hassan Matt Owens revealed that Usopp's fight with Chuu had additional scenes which included Usopp's inner struggle and him nearly giving up the fight, like he originally did in the manga. Those scenes had to be cut when they received news that One Piece was going to be an eight-episode series and were forced to cut off material in order to keep to the alloted time of eight episodes and still make sense.
