They've had a game in preproduction since 2010 which is almost certainly Fallout 4. Bethesda has only worked on 2 IP's in recent years, and Bethesda has said in the past that they didn't buy the Fallout license just so they could make one game and forget about it. They could be working on a new IP or the next TES, but it's doubtful. We have tweets from ThreeDog voice actor Erik Todd Dellums saying that ThreeDog may be returning soon. We've got Inon Zur, the composer of Fallout 3 and Fallout New Vegas, hinting back at PAX that his next game will be announced on May 15, which also happens to be a week before the rumored date of the Next Xbox unveil (Microsoft bought the timed DLC exclusivity rights for Skyrim and may do so for Fallout 4 as well, and they'd likely show it off at their unveiling in that case, since Sony showed off multiplatform games they had DLC exclusivity rights to at the PS4 unveiling).
Personally, I'm glad they're moving on to their next game. I liked Skyrim, but I like Fallout even more and I'm so ready for Fallout 4 after replaying Fallout 3 recently.