I think it is going to be either FR or Exandria. Regarding Exandria the book is probably slightly a year out at this point, and the 3rd campaign will probably start in September, so about a year before the new book would hit the shelves, so I am not convinced it is too soon. I suspect though it might be FR, because of the reasons others have put forward. I can't see it being Eberron, it has already been revisited after the WotC book, and another revisit is in the works, neither were connected to WotC.