My guess is that it'll start with another telling of the birth of the rebellion and will take place somewhere around the middle of the 20 year timespan between Ep.III and IV. We'll get to see Vader, Tarkin, Emperor, Mon Mothma, Ackbar, Han, Chewie, and Boba. I think we'll also see a young Leia with Bail, maybe even see much more of Alderaan. Early X-wings and TIE's, and bunch of other cool stuff that will bridge the gap from Ep.III to IV. I'd even go as far to say that we'll probably even get a glimpse of Ben and Luke on Tatooine. Along with all of that OT goodness, I'm thinking that we'll also see characters from TCW returning. It'll be awesome!