I've had a creepy thought about Sylar.
He's essentially a polar opposite/dark-mirror reflection of Peter, right? Peter is a twin brother. Sylar should have a twin, too. Of course, it wouldn't be an over-protective, stable figure like Nathan. Oh, no...
"That sound, in your heart. What is it?"
"Oh that? That's my twin brother Steve. See, I have the power to absorb and assimilate genetic material, so in the womb I absorbed my brother. It's actually a process called 'parasitic twin syndrome', or 'twin-to-twin transfusion' that happens in the real world, except in my case Steve is still alive, and he makes me evil. He's got this wierd power where he knows how things work. Now, hold still while I eat your brain."
Gross, right?
"Steve" could even be the driving force in Sylar; basically just animating Sylar's body from inside. This is how he seems to be able to resist injury/come back from the dead. Kill the body, and "Steve" just repairs and re-animates it from inside.
To quote George Carlin: "These are the thoughts that kept me out of the good schools"...