No plans for a downloadable expansion for The Elder Scrolls V: Skyrim have been announced yet, but a trademark filing spied at the end of February is believed to tip off its name or part of it anyway: Dawnguard.
ZeniMax Media, the parent company of Bethesda Games, filed the trademark; "downloadable software" is listed in the categories of products it covers.
VG247 notes that, in the city of Dawnstar, Skyrim features a quest called "Pieces of the Past" that concerns a museum dedicated to The Order of the Mythic Dawn. Dawnguard may refer to that. Others have speculated it could be a trademark for another mobile title.