In the process of developing a video game, many ideas are discarded as unworkable for one reason or another. However, if the idea makes it far enough into the development process, aspects of it can be still be found within the code of the released games, or it may show up in trailers or interviews with the developers.
The following list covers much of this removed content. It is not comprehensive, and focuses on the more prominent elements.
The Trio's home world
The Trio's home world was not originally set as Twilight Town but was removed due to game development. Carona and Threads of Fate characters A world based on Square's Threads of Fate and its characters were scrapped from the game due to no having connection in the game's main plot. A Keyblade based on the world was also removed.
The only characters announced were Rue, Mint, Claire, Ruenis and Ruecian.
Arachne and Spider Riders characters
A world based on the anime Spider Riders and its characters were scrapped from the game due to the fact that Spider Riders is not of Square Enix or Disney.
Kim from Disney's Kim Possible were originally set to appear in Radiant Garden but were removed due to story development.
Serah from Final Fantasy XIII was originally planned to be with Lightning (SoM) but was removed due to creation of Final Fantasy XIII-2.
VII's original name
VII was supposed to be named as Xail having a reverse Nobody anagram of Lia. This will be changed due to unknown reasons.
Nexus's original role
Nexus was supposed to be a Nobody existing while Suné exists like Roxas existing while Sora exists. This was changed to differ the story of Suné and Sora. Nexus was also originally set as a boss and a Dimension Link.
An artwork of Suné in Valor Form was made during development. The artwork contains Suné in a red palette swap of his clothes with a fleur-de-lis symbol on his sleeves and pants. The Drive Forms were eventually replaced by Command Styles.
Summer's End Keyblade
A Keyblade called Summer's End was to serve as a joke weapon but was replaced by the Umbrella.