Disney references are a major part of Kingdom Hearts. I just don't see how they can add him without referencing Donald & Goofy, who're part of the main trio.

I suppose just having the KH-originated characters as spirits and redesigning Sora's keyblade to remove the Mickey Mouse reference could work, but I just don't see Sora happening.

Believe me, it'll be truly epic if the madman Sakurai did manage to get him in like he did with Banjo & Kazooie.

I would love to see a spirit battle that is just Orginization XIII.

You could have Marth with a beam sword for Xemnas
Wolf for Xigbar, Corrin for Xaldin, Link for Vexen, Ganondorf for Lexaeus, Robin for Vexen, Ike with an ore club for Saïx, Incineroar with a boomerang for Axel, Greninja for Demyx, Mr. Game and Watch for Luxord (I'd say Joker, but we don't really see DLC fighters in spirit battles.), Lucina with Death's Scythe for Marluxia, Zero Suit Samus for Larxene, and Pit and Dark Pit for Roxas and Xion.