Let the speculation commence. In an under-the-radar move at the end of February, ZeniMax filed for a trademark on the name Dawnguard.
While the name could be for anything, we do know that Bethesda also owns Redguard, a group of humans from the Elder Scrolls games. There is also Dawnstar, which is the capital city of The Pale in The Elder Scrolls V: Skyrim. It's a stretch, but who knows?
The only clue in the listing is that Dawnguard is for "computer game software for use with computers and video game consoles; downloadable computer game software offered via the internet and wireless devices."
This could be new DLC content for Skyrim, a new name for or group in its upcoming unannounced MMO, or something totally unrelated. If we figure it out, you'll be the second to know.