They'll never do this, so I feel no fear in letting the idea get out there:
I would love to see a character who is, in effect, John Carpenter's The Thing.
To give you a Heroes-universe explanation, a disease that ate a Sylar-like character. The Disease would feed on healthy tissue by splitting it's body... sort of the way Beetlejuice did with his face when asked, "Can you be scary?" The creature splits apart, surrounds the victim in tentacle-like intestines and organs and cocoons the victim, and then absorbs it... effectively the cannibal I thought Sylar was once.
And then slowly over the course of a few hours, the victim begins emerging as the older tissue dies away and is discarded, until the creature thinks it *is* the last victim.
That, to my mind, would be something scary enough to make Sylar run the other way. He couldn't kill without using a method that could possible kill himself (like burning or acid). And would give Sylar an excuse for another heroic turn.
could probably still just absorb the power probably not. that bit with Elle was a fluke. it's not something he could probably do with anyone else because he's unwilling to open himself emotionally with jsut anybody...
...although, thinking he's Nathan now might change that.
There was a guy who could change his arm into steel the chapter before last.
Nah, he got the power the old fashioned way with the shifter, just didn't go through the front so it wouldn't look like he killed him. He also didn't get Nathan's power, Sylar can sorta fly using his TK.
A character who can assume, mantain and control a gaseous form.
Sylar comes up to them to steal their power (the whole head cutty open thing), and the potential victim turns to gas, rushes at Sylar, and gets inhaled, cutting off the flow of oxygen to Sylar's lungs.
That lasts for about 30 or 40 seconds of Sylar rolling around on the ground, choking, when the gas exits him, says "Don't try that again, or I will kill you." and leaves.