This may be a bit too simplistic, but I'm somewhat worried that the manner of how the storyline evolves in season two will be too similar to season one, if they start over with a new cast.
If it's just another rendition of discovering one's own powers -> realize there's a threat -> meet other heroes -> work together to stop the threat, then that'll quickly become too fomulaic. That's part of the reason to keep the same cast in a show, otherwise you tend to retread the same ground, plot-wise.
Of course, I'm confident Tim Kring won't let things become that stale, but it is rather irking/worrying that we'll be back to square one in terms of getting to know the characters, at any rate. I'd rather they kept at least a few of the main ones, and let most of the rest of the cast change around them (similar to how it works on each season of Boston Legal and, if what I hear is correct, since I don't watch it, 24).
I look forward to the season one cameos in season two, in any regard. I bet Peter will survive, and become the big gun that they pull out only on rare occasions.