 ::Boba Fett seems like a good addition to me, but I think Hiro Nakamura is far more of a breakout character for Heroes, and you just glossed over him.  Hayden Panatierre may be on the covers of magazines (she is a photogenic young lady), but almost every article I have read regarding Heroes has mentioned Masi Oka, and a fair number of them have been about him exclusively.  That said, I am not sure that either of them are true breakout characters.  While the Petrelli storyline is central to the story, both characters have had a great deal of screentime from the very beginning, and I don't think they were ever intended as minor roles.