We're 100% getting an expanded final mix style thing that's well beyond the scope of the past ones OR a Mirage Arena, Blood Palace style combat platform for this stuff.
It's easy money, they have the models, rigging, and moves all done you just need to shift control.
Eh, not quite that simple. I hope so! But just because they have a ground combo string, a couple of special moves, a non-aerial spellcasting animation, a running animation, and aerial/ground hitstun animations doesn't mean they have *all* the assets right there (jumping, walking, aerial combo strings, recovery, blocking/countering, et cetera).
Aqua and Riku are clearly *playable* but both somewhat barebones (especially Riku, who doesn't have any Command Styles/Forms at all, just Dark Firaga).
Now, KH2's animations (with playable Sora, Roxas, dual-wielding Roxas, plus separate full combo/finisher strings and stuff for every one of Sora's Drive Forms) can certainly be mined for *some* of this stuff, as can (to a much lesser degree) Birth By Sleep.
It's something I really hope to see done, and *some* of the work has been done to figure out how each of these characters would play, but that doesn't mean it would be as simple as flipping a switch - a pretty good amount of additional animation assets would be required.
Is there something to start with? Absolutely yes. But I'd rather see the version of this where they take the time to flesh everyone out to feel truly good to play with, not the easy way of doing it.