Having Disney characters without Sora present would feel weird, because he'd very likely be mentioned time and time again throughout the game. So I'm thinking maybe they will use characters they didn't use before, since KHIII isn't really about him. (This is all assuming that it takes place in the future.)
Assuming it takes place in the past, well, some of the Disney characters wouldn't even be around. And I personally think it takes place in the past, so why not use Studio Ghibli characters? I've seen some of their work, and it looks pretty good. It may actually fit better in KH3 since, even though it was mentioned before that we can't assume it's dark because of the trailer, it's a Keyblade war, right? If so, then it will most likely be a bit darker than KH2 was. So to have Tarzan swing from the trees and kill *insert enemies here* without Sora right behind him would seem out of place, since Sora's one of KH's trademarks. Or maybe that's just me.
I was thinking, before reading this, that they might use Pixar characters, really. And now that I think about it, has it even been confirmed that the 3 characters we saw in the trailer will be the main, playable characters? Just because they have Keyblades doesn't mean anything. They could die for all we know, and they Keyblade could go to someone else.
So, either was you put it, using Studio Ghibli characters seems justifiable. I think I could live with that.