The way I see it, KH3 is going to be a giant mixture of all the characters we've seen, going into one final adventure and epic battle. We've got:
Sora, Riku and Kairi: Sora and Riku are both wielders, and Kairi could potentially be, and if not, I suspect she'll still be along for the ride, due to various hints in KH2. I would imagine if SE keeps up the main game tradition of having a playable trio, this would be it. However, I feel like Nomura would also take bits from BBS, like making each character playable, each with their own style.
Mickey, Donald and Goofy: I'm betting KH3 will be the first game where Donald and Goofy aren't fighting with Sora, but are teamed up instead with Mickey. I don't envision playing as these three, but I imagine them being quite a playing force in the story, meeting up with the main group at several points and helping in crucial parts of the story.
Terra, Ven, Roxas, and Aqua: This is where things get curious. I think it's pretty easy to assume now that we'll be seeing at least hints of these three in the next game. I believe we'll be fighting Terranort's somebody, who, to make a more formidable villain, will somehow have regained his memories, remembering how to wield, and using his ridiculous powers to spell doom upon everyone. If Nomura's logic is correct, he'll have returned to his original form, though that would still be MX in Terra's body. I think Lingering Sentiment will make an appearance, if not just for the sole fact that he says he will at the end of BBS. He could even end up being a major character. I think he'll know something about Ven being inside Sora, and somehow, Roxas will be freed, and through him, Ventus will finally awaken and be back. I think this would please all parties. Ventus would return with his identity and memories, while retaining Roxas' skillset (dual wielding, etc). Terra would be there in spirit, and I can't really comment on Aqua, since I'm not sure how her scenario ends. I think, in the end, Terranort will finally be defeated, and through Kingdom Hearts or something, Terra will be able to reunite with his body and find his redemption.
And then of course, there's other characters here and there, most notably Organization members. I think Axel will definitely return as Lea, and perhaps Isa and Braig. I think at this point, most of the Org members don't really have significant staying power. I think Lea Isa and Braig are the most likely. Lea for sure, since everyone seems to like him to much.