I don't think a KH fighting game would need the Disney cast outside of MDG, and even then it might not need them. Original members of the KH cast are the main fighters of the series, and the Disney characters could easily provide a summon-like function. Also, don't forget that Disney does have rights to the original KH characters and can decide if they want to see them fighting either. I think that point is kind of moot though since they fight each other in-game lol xP
Also, I'm confused how you can see Ariel fighting, but not Jack, Beast, or Peter Pan lol? Honestly, I think it would be weird to see Disney characters with a given moveset to fight with, so I think their roles would be better as partners. Kind of how in Duodecim, Aerith was an assist-only character, I think Disney characters would fit that role perfectly.
Honestly, I don't think a few hours would be enough to cover all the years Aqua has spent in teh RoD. Not to mention that it would be worse to have the concept of BbSv2 be demoted to a tutorial.
When it is released, I just want to know how spoilerific it will be since I think it will end up be full of them. In which case I'm going to have to wait all the way until August in order to read the full thing (after I beat the US game once).