I don't care what anybody says, I think they're going to have to put Lightning in some Kingdom Hearts game, eventually. I've told a few people about this already, but her Lightning Returns story is actually perfect for this. I mean, you changed "soul" to "heart" and "chaos" to "darkness" and you're pretty much done. I would imagine that if she were included, she would have to travel between worlds looking for somewhere to release the Hearts of the people she saved on her original world (now swallowed by Darkness), and Sora would meet her randomly on the Lanes Between or actually IN a world. It would be cool if you happened across her on random worlds after you've already met up with her, and find her surrounded by Heartless and Nobodies. You help her fight them off, and then she'd give you some materials or something and take off for a new world.
I would also like for there to be worlds you don't have to visit in a pretty big way, which could work well with the idea above. It's all well and nice to have a set of worlds to go to that are all somewhat relevant to the main plot, but we need more worlds to explore for the fun of the story from which te world came, or for more original stories in the fashion of Traverse Town and Radiant Garden. I think it would be cool if finding a world where Lightning can free the Hearts she's saved to become people again was a completely optional thing.
I would also like to see physical expansions to worlds that already exist. More areas of places we're familiar with, I mean, as well as the true opening-up of worlds we see in scenes but never really get to go to. (I'm looking at you, Destiny Islands) It'd also be nice if, as you save people like Aqua, you could switch over at any time. The style of Birth By Sleep was nice, but those stories all started at the same time and you couldn't switch without starting a new game. Dream Drop Distance was cool for letting you switch between stories on the fly, but also a little annoying in that the switch was often unavoidable. I would like a healthy balance.