VG247 have picked up on
a recent trademark filing
by Bethesda relating to the name Dawnguard for an upcoming videogame-related venture and as The Elder Scrolls series tends to have the most liberal use of the word "Dawn" out of all Beth's existing franchises speculation is all currently focused in that direction.
Realistically, it could be anything (or even nothing at all -- a mere name doesn't create a product), but the most credible guess that the Internet braintrust hsa to offer seems to be that it will be the first downloadable content for Skyrim. VG247 is leaning towards it having something to do with the Mythic Dawn -- the occult faction that helped open the dimensional gates in Oblivion -- but Skyrim has plenty of other references to Dawn, so if it is Skyrim related, our money is on it having something to do with Dawnstar
, one of Skyrims Northern coastal cities and perhaps some kind of look into its past.
That said, we're really just taking shots in the dark like everyone else, but bring on more Tamriel Northlander adventures Bethesda.