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.