Either TES5 or Fallout 3. They keep saying they want to do another ES game this generation, and if that's true, they'll need to get on it soon. And Fallout 3 is already going, so I expect there'll be an announcement soon... however, they've already announced that they're working on it, they just haven't released anything real to the public yet. So my guess is this is TES5, since that would be more of an announcement of a new game.