Apparently Zenimax recently filed a trademark "Dawnguard" This website thinks it's about the Mythic Dawn. It would stink to revisit Mehrunes Dagon's realm again.

http://www.computerandvideogames.com/340862/is-dawnguard-the-first-elder-scrolls-v-skyrim-dlc/

Could it be the name of the mmo? Or just an entirely different game?

Another one of these threads?

http://www.gamesas.com/topic/1352213-new-skyrim-dlc-is-probably-called-dawnguard/page__hl__dawnguard