They've confirmed that Lightning will at least make playable appearances (what that means hasn't been elaborated on, but I'm guessing we are going to see her with her own subplot as the game progresses until Serah and her are finally reunited).
Snow, Hope, and Vanille (in some capacity) are confirmed appearances (at least).
We also don't know the fighting capacity of these characters. I'm pretty sure that the concept behind Lightning is that she's a l'cie again, but blessed by the goddess rather than the fal'cie (and willingly became so), so she's not facing a future of being a cieth. I'm GUESSING that the enemy dude is similar, but is a l'cie blessed by the god Lindzei, who created the fal'cie of Coccoon and is a Loki-like figure.
Whether or not Hope and Snow have followed the same path has yet to be seen. We know that Snow left to complete some mission, promising Serah that he would be back.
I'm guessing that at least Hope has followed a similar path as lightning (even if he doesn't know where she is), simply because it's hard to imagine why he would choose to live away from everyone as he seems to have been doing. And that's what gives me hope that he's become a normal human being, rather than a whining freak. I'm imagining him as someone who has become a sort of sentinel. :P
I'm honestly excited for the game. It seems to me like they have taken every complaint people made about the first one and really thought about them--we have a more interactive combat system, towns, and a more open world to explore. Plus, the game is far more story-driven than the previous one is, only having two characters (one of whom has already seen a fair amount of development).
The only thing I hate is the outfit Serah is wearing... I mean, c'mon...
Anyways, you all are horrible, @#%^ed up people
Never underestimate the healing power of a massive dong.