No, I think Nikki's dead. I think Ali will be playing a new character. I think.
Season two (though I basically liked it) lacked a great deal, IMO, because it's two best "couples," Hiro and Ando, and Peter and Nathan, were apart for too long. I liked Noah's continued development and the introduction of Elle, but Mohinder bored me to tears. He didn't used to, but he was woefully misused IMO. I am excited with spoilers I've read, because the Peter/Nathan Hiro/Ando interaction problem seems to have been solved and then some. I love the stories planned and the ramifications they'll have.