it's not that unlikely. nomura seems like he wants to give people a way to play their favorite characters.
I think it'd be cool to get a sort of multiplayer co-op side campaign. saving some worlds while sora's busy elsewhere, or whatever, and you'd play as any character who you've unlocked in the main campaign.
start out with Sora, Riku, Kairi, Lea and maybe Mickey? Then as you go you'd unlock Aqua, Ven, Roxas