Yep, I think it's called Dreamspring Canyon and is the Dawnguard Fort place, looks marvelous
I think it's called Dayspring Canyon. The vampire castle is located northwest of Solitude which that new location is Soul Cairn (a new plane). Dayspring Canyon, to me (judging by the trees), looks like it is located somewhere near The Rift Hold (a previously unaccessible location).
OP:
Nobody really knows much more then a few things about Dawnguard. We are all kind of in the dark about this. It will add new creatures, higher level dragons (legendary), 2 new locations as well as 2 new forts to have (own?), new questline, new side quests, new dungeons (one having nothing to do with Dawnguard), new weapons, new armor, new perks (and perk trees). That's just what we know, who knows what else lays in wait come this Tuesday! As for future DLCs, if history repeats, we should get an expansion that resembles Bloodmoon (Morrowind) and The Shivering Isles (Oblivion), which were their own island filled with new towns/cities, quests, creatures and landmass. This is just the beginning of the DLC and won't be the last. They'll end up saving their best for last.