The new expansion will take players to Praetoria, a realm occupied by both the bad and the good that is ruled by Tyrant, the "evil incarnation of the game's main protagonist, Statesman." Praetoria will feature a new alignment system that will allow heroes to become villains, and vice versa, based on the decisions they make. This means that heroes who become villains can travel to the Rogue Isles for the first time, and the same will go for villains-turned-heroes, who may now experience Paragon City.
There's no word on release date or price for City of Heroes Going Rogue right now, so hopefully E3 will swoop in and save the day. In the meanwhile, you can entertain yourself with a barebones site for the expansion here.