Bethesda tweeted they'll be revealing the mystery tomorrow.
All 3 Vine teasers:
Some people think it might be a Wolfenstein reboot, but I think all the German-y stuff is an opaque reference to Shinji Mikami's (RE:1-3, Dino Crisis, Viewtiful Joe) new survival horror IP, codename Zwei, which obviously is of German origin.
As much as I'd like to see Fallout 4 sooner rather than later, if this does turn out to be Zwei, Mikami's new IP-- in a genre he defined-- definitely has me excited.
UPDATE: Damn, that was fast. A little after the final teaser video went up, French site Jeuxvideo lets the zombie out of the bag by saying "New game from the creator of Resident Evil tomorrow" in reference to an apparent exclusive.