Well for DA 3 your probably going to at least start in Orlais. Its where Bodhan and Sandal were heading. If Alistar was made King there is a scene in DA 2 where he talks about how some Orlaisains want there old territory back. My guess since the DA 2 expansion was supposed to be called the Exalted Marches is that the quinari have attacked in force. Since Orlais is the religious center of Thedas and home to where ever Divine Justine happens to sit, I'm guessing that the March would of started there, much like our Crusades started in Italy. The Quinari would see this as the perfect time to strike at the heart of "inferior" culture. The game would open with the Divine, Empress, and possibly Leliana trying to escape the city. This would give them a chance to re-introduce the ability to play as different races. You just start as some shmuck who gets caught up in it. A little cliche but hey, it works.
I would also like to see some new teammates, especially a tevinter magister. Through the first two games we have been told they were evil and responsible for the darkspawn, but the DA 2 DLC, i cant think of whats its called right now, cast doubt on this because Cor-whatever the hell his name is says that the city was already black when the mages got there. You could make this interesting too. This magister could be just starting out and ready to make it in Tevinter and your actions and convo's could sway him to use Blood magic or stay with more accepted practices. A summon heavy Demon tree of magic would also be pretty cool.
Anyway just some thoughts and ideas, but when it comes time for DA 3 i know i will be there at midnite. lol