Well, there's a mod for Oblivion that adds Morrowind, and I remember seeing a project mod to add Cyrodiil to Morrowind. I'm sure that type of pattern will continue on in Skyrim and in future TES games, so long as a Construction Set/Creation Kit exists for the PC.
As far as an actual game released with all of Tamriel, I can't imagine that ever happening again. It's just too much stuff and it would take way too long to make.