Not being a big Kingdom Hearts fan, I don't know if this flies in the face of the mythology of the franchise or whatever, but what if he was referenced in name only as one of the suspected but long-gone creators of the worlds / environments within the games? It would also be an indirect reference to the people responsible for the rest of the characters and locales featured in the Kingdom Hearts games.
Alternatively, you might have a separate "Main Street USA" world in a future game, and he'd be the city planner of the place, a reference to Disneys' big role in planning the initial layouts of the first Disney theme parks.
In any case, I can't help but see him as a minor or unseen character at most. If he is in a future game, I don't think there ought to be any hints that he's actually present until you manage to find him in some obscure back-room or corner somewhere. Featuring him as a major, story-dependent NPC would take out all the fun and subtlety of discovering Walt for yourself.