This is just a theory i came up, but in Kingdom Hearts Birth By Sleep, Venitas said there is thousands of other worlds in the Kingdom Hearts universe besides the Disney and Final Fantasy worlds, each with their own KeyBlade wielders. could SquareEnix use this to add a custom character in a future KH game? unlikely but still fun to think about.
But if they do add one, it would probly be like the one in White Knight Chronicle or Dragon's Dogma. you create your own battle partner and even have the option to play as your created character. choose your characters gender, age, hair style, body size, clothing, and weapon you use. the story would still be mainly focused on Sora but you had the option to play as your created character just like in White Knight Chronicles.
But like I said, its unlikely to happen but still a nice thought. But I see the custom character working better as an Online type of thing, or a separate game like Halo Reach. you get your own customizable character, and you have a story mode to play. but not a full RPG. (Because if it was, I know Disney would not be happy with you killing Cinderella or joining the villains and destroying the universe) it would be slightly RPG, but more like Dragon's Dogma. you couldn't go on crime sprees, you couldn't kill someone because they made you mad, and you couldn't steal. you were on a quest to save the world, and choosing what side missions to complete would result into different endings. like in FallOut: New Vegas.
If they did do this game, the story would ether be a prequel set a few months/weeks/days before KH1, or it would be in an alternate universe where Sora was never born and you take his place but the story plays out differently depending on your actions. you even get the chance to kill the story's main villian early in the game, but if you do that a close friend to your character is some how corrupted and you have to fight him/her at the end. although you're not given the choice to be bad, your choices can still result into bad endings if you're not careful.
But as i said before, this is just a theory.