We’re always waiting for the Ausiello spoilers, so here is the bombastic one, regarding Sylar (since it’s spoiler, click on the link to read it):[spoiler]
As expected it seems like the major Heroes death will be at the hands of Sylar in the Season 3 Finale Episode 3×25 An Invisible Thread.
EW’ Ausiello confirmed this information today and based on prior spoilers it seems like Sylar will indeed use his shape shifting ability to accomplish this. Of course the most likely scenario is Sylar killing Nathan and taking his place. Here is Ausiello’s full post:
Question: Any news on Heroes? Maybe something on the season finale? –Dennis
Ausiello: Sylar kills a regular Hero that’s been on the show since Season 1. And it sticks. (Source)