A trademark filed by Zenimax Media around the end of February, and totally missed by us, may possibly hint at a possible expansion plan for The Elder Scrolls V: Skyrim.
Called "Dawnguard" the filing states the trademarks is for "computer game software for use with computers and video game consoles; downloadable computer game software offered via the internet and wireless devices."
Various outlets have theories as to what it could be, from DLC for Skyirm to another mobile title akin to Dawnstar.
Dawnguard could be related to the The Order of the Mythic Dawn, a sect which was responsible for the Oblivion Crisis in The Elder Scrolls IV: Oblivion, and also behind the assassination of Emperor Uriel Septim VII and his sons. In Skyrim, a quest line option in Dawnstar starts a quest called Pieces of the Past for a museum dedicated to the order and it leads to the recreation of Mehrunes' Razor from the Daedric Prince of destruction, change, revolution, energy, and ambition, Mehrunes Dagon.
The Dawnguard could indeed be something similar to Dawnstar's release, or something for Skyrim. Or it could be for something entirely different and have nothing at all to do with Mythic Dawn.
Another recent trademark, Redguard, could be more content for Skyrim since it is a race prominent throughout the series, or something akin to The Elder Scrolls Adventures: Redguard.
It's a mystery and will remain so until Bethesda decides it's time to announce something.