As they talked about how Garrosh is doing something beneath/under Orgrimmar, it wouldn suprise me if the Raid will start in Orgrimmar and then go down into Ragefire Chasm and we will fight him there. They could change Ragefire's layout, make it bigger or even completly change it except for being underground. It wouldn't suprise me.
As for bosses, Gallywix and Magatha could be bosses that appear. Then we will probably get a few bosses that are just there, but have no or only little lore prior to the raid. And then Garrosh as a final boss, yeah.