The answer to that question will contain huge SPOILERS ahead...
And we start with...
She discovers a new ability, as well as new secrets. Claire's world will be very different from here on. In the season premiere, she and Sylar will go head to head.
He won't be himself...and nope, that's not a season 2 flashback when Peter didn't quite know who he is. This time, he really isn't quite himself.
Seems he's either a walking dead or sees dead people, the first one being Mr. Linderman who died in Season 1. The Petrelli brothers will discover not one, but two Heroes are related to them. Mama Petrelli is going to be a very significant figure this season.
We last saw her in that bomb blast. Did she die? Or did she resurrect with a third personality? Tracy, Nathan Petrelli's political adviser? Does Tracy have super powers? The answers are in the season premiere.Tracy supposedly had a twin.
He'll go after his nemesis, Speedster/Daphne, who takes something that belongs to Hiro's family.
What is up with Parkman? He's lost somewhere...and that is all we know.
He will acquire super powers by injecting himself with the serum he created to cure Maya. Unfortunately for Dr. Suresh, he doesn't know how to control his powers. His character will be ala Jekyll/Hyde. Also Jekyll/Hyde? This new show from NBC.
He and Claire Bear's daddy are going to be buddies. Whether that's a strategic move by Mr. Bennet, is what we're going to find out in the season premiere. In the future, Sylar is living in a suburban house with his son Noah, who is also gifted. Noah's mother is "silver-tongued". Oh, in the season opener Sylar slices off someone's head, big time. If you wanna know who, follow this.
Sources: E!, TV Guide, Spoilerfix