So I was reading up on Kingdom Hearts' plot recently, don't ask me why...
But the whole Light and Darkness dychotemy balance thing is reminiscent of Final Fantasy III and IV to me...
So I'm thinking there's going to be something that connects Sora to Aqua directly this time around. Especially given the fact they have to wake Ventus up and get Terra restored again.
So we might see the likes of Cecil, Kain, the Cloud of Darkness, the Onion Knights, etc... maybe even the characters from Final Fantasy Dimensions.
And given that Kingdom Hearts is an actual "place", players might actually get to go inside and visit it.
As for mainstays like Cloud, Leon, Sephiroth, etc... they'll probably get involved somewhere along the line.
Heck, we might even see Lightning....