I'd like to leave Tamriel... not to do a completely different continent for a dlc but to visited one of the islands or set of islands between continents, fo example between Tamriel and Atmora .. further north than we have been before and closer to the original home of man.
Although the name Dawnguard suggests something from the east, perhaps something has risen after the eruption of Red Mountain. I hope they bring back the cliffracers now that they've got improved animations and behaviours for flying creatures, it would definetely be good to see some none "boss" type flying creatures.
Or possibly the Akaviri (which is probally unlikely but i always hope for them to be involved everytime there is something new in TES world) however they were the original Dragonguard and with Dragons back it would be a good time for there return.
A new continent, large island or set of islands would be most intriguing. However, I'd be very nervous about going to Atmora. Everything I've read in the lore makes it sounds like a terrifying place. In fact, before sending any of my characters there, I'd like a Fallout - type option of equipping him, at a minimum, with a man-portable Ma Deuce with 1000 rounds, a flamethrower, and three tactical nukes.
Seriously - Atmora sounds like a real freak show, and the above loadout would be a minimum, IMHO.