Eh. I have mixed feelings on it. I think it'd be better to get an all new cast or at least a new main character. Someone that hasn't been in the series until now. But I also think Sora is the character you think of when someone says "Kingdom Hearts" so I mean.
I feel like Marvel and Star Wars could be a strict no from disney. But ya never know. Pixar I think could work. And if we get a FF world I just really want the gold saucer.
Maybe, but since they come out with a new game every year if they needed another year to work on KH3 they could probably release some other game like Kingdom Hearts Dissidia or Theatrhythm.
I would love to see them. I think if Marluxia and Larxene appeared they'd just sort of be there. They wouldn't be any of the thirteen darknesses considering they planned to take over the organization. Luxord could be a darkness. I'm really not sure about Demyx.
KH3 isn't confirmed for Jump Festa, so I'd say until at least E3 2014 we won't be seeing anything except maybe an interview with Nomura every now and then. Honestly though I'm not expecting anything until TGS 2014.