I can't imagine how MASSIVE it would be if they made a game covering all of Tamriel in the detail they have done with Skyrim but OMFG that would the best thing evah. I'd give up a lot for that: friends, eating, hygiene, multisyllabic communication with other IRL members of my species...
I think maybe a multiregion game might be the most I could hope for right now, like a game that has quest lines that cover Hammerfall, Daggerfall and the Summerset Isles? I would really like to take a break from the snows of Skyrims and get lost in a place like the A'likr Desert, and a game that includes sea travel could potentially be pretty awesome. Sea monsters, anyone?