In case you didn't hear, some of the leaked audio files indicate we might be returning to areas from D1 like the Cosmodrome, the Hellmouth, the Ishtar Sink, Meridian Bay, the Dreadnaught, and the Reef. Well, remember when some parts of the Cosmodrome returned in RoI (as part of the plaguelands) and its appearance was given an almost complete overhaul? If the returning areas are given that kind of treatment, I'd consider going back to them a good thing.
We already return to two older locations. We can go to mercury (on a PvP map) And we can go to mars (on a PvP map)
Reef, Dreadnaught, and maybe Meridian Bay. I'm not sure about the Ishtar Sink or the Hellmouth yet.
Well the best theory i have seen was a video on YouTube in regards to something Luke Smith said. Apparently if you look at the end cut scene when the light spreads out, the places it visits is the order the DLC for D2 will be in....So Mercury (Osiris), Mars (Rasputin/Warminds), the the Reef and then either Saturn/dreadnaught and then the Darkness right at the end (IE D3). If this is true then i'm sure certain places from D1 will be in there