So, upon some thought, I realized that Noel Kreiss and/or Caius Ballad could also work, perhaps. No doubt in trying to save Yeul, though they could also go down the "Noel just wanting to save the future" route if they really wanted to, I suppose
I also did like the idea of the Cloud, Tifa, and Zack idea being rounded out somehow, as well as Hades using the Titans, that someone here mentioned earlier
I'm pretty sure that the Titans
will be in KHIII, and that they will be Hades' main ploy, but I see no reason why a FF character can't be involved in this all, either.
Especially since, if we're going by the movie, someone is going to need to distract/trick the characters so that they can't react to the Titan threat at first. No one would trust Hades at this point, but maybe a new FF character, working for Hades, could convince Hercules to give up his strength or something?
Edit: Still not over how amazing that fan Lightning edit is^_^
Edit Deux: But back to my idea of it being sorceress!Rinoa, I'd be interested to see how Nomura would handle that in KH. I feel like it would be much like FFVIII, where most people hate sorceresses, because we already know from Cid that he ain't too crazy about witches, seeing as how the last one he met destroyed his whole world and forced him to escape with Leon, Yuffie, Aerith. (And possibly Cloud and Tifa, and Rinoa, if a story like this is to happen?) So I feel like there could be some drama there if Rinoa were to suddenly develop powers.
Though, of course, like in FFVIII, Squall would soon decide that he doesn't care