I was thinking about this. I think the most likely option will be that some form of pass will open up somewhere and we'll be able to get past the mountain range that blocks us off from the back of the rift. I dont know how they'll do it, but they could easily have a land mass shifting, some form of rock avalanche or a dragon knocking part of the way clear?
As for the bit around the sea of ghosts, i have no idea. I'm quite happy just to wait and find out to be honest.
Yeah, I just hope new area added is at least around the size of one entire region. There's a suspect screenshot I've seen on G4, that shows what appears to be a Basin that doesn't have any familiar landmarks. Initially, it looks like Eastmarch, but the direction is all wrong, the Mountains are sloping down to the East or West (Sun setting/rising, can't tell really), and things like Bonestrewn Crest, Mistwatch, Windhelm are all absent.
My guess is, if you've explored the actual borders of Skyrim, near Stendarr's Beacon, there's a gate that uses an invisible wall to block advancement. I'm betting that's where we'll get to Fort Dawnguard from.
http://e3.gamespot.com/image_viewer/6380876/the-elder-scrolls-v-skyrim-dawnguard-screens/11/#site_nav It looks like he's coming in from the Southeast of Skyrim maybe into this new looking area. If anyone can pinpoint a known Skyrim Landmark in here, let me know, 'cause I don't see any.