Did you know that there were plans for a third battle against Riku before the climatic battle against Ansem? The place where you were to fight Riku is a place called "Crumbling Island", one piece of concept art exists for the area.
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Within the files of Kingdom Hearts, people found an unused summon called "Bahamut", Bahamut is a very powerful summon that has appeared in alot of Final Fantasy titles. Selecting Bahamut will cause the game to crash, however.
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The original logo for Kingdom Hearts was extremely different form the original.
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Kingdom Hearts Chain of Memories started off as a drastically different looking game. This shot implies that "A dive to the Heart" was planned to be in the game, There are also huge aesthetic differences, including still pictures of the characters on cards.
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Deep Jungle is a world based off of the Tarzan movie made by Disney, it only appeared in Kingdom Hearts 1 due to Disney being unable to get the rights to Tarzan in future games.
However, Deep jungle was going to be in Chain of Memories, even a card for the Jungle king exists in the game.
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Xemnas, the final boss of Kingdom Hearts 2 was planned to have two additional forms called "King" and "Kentauros". They look as though they were built up of the "Dark City", which make up The world that never was.
Thats all for now!