According to Spoilerfix, 12 new characters will be added to Heroes Season 3. Amongst them would be Brea Grant who plays hero/villain Daphne or Speedster. She is set to be Hiro's arch-nemesis.
Another new character added to the season is Benjamin Knox Washington, played by Jamie Hector.
Knox is rumoured to be the main villain of this series. SpoilerTV has revealed that Knox has returned to New Orleans with "plans to reclaim the criminal territory he used to run before he was sent to prison. This recurring character will appear in episodes thirteen and fourteen, and possibly more to come".
Finally it is almost certain that Knox is the leader of the gang that kidnapped Monica, so he will her and Micah down to tie up loose ends.
A few other spoilers:
1. The new season's plot will focus on something other than The Company this time.
2. We will see more of the younger generation of Heroes.
3. Micah's story line will come to an end.
4. Sylar's character will be more badass than ever and he will kill a main character.
5. Ali Larter (Jessica / Nikki Sanders) will return to the show, but her story line will end in the coming season.
Heroes will return this fall.