I think if Warhammer ever made a game it would be good as they already have their races and lore etc and I guess they could possibly have the funds for a major design team.
Umm.. Warhammer as a company or game? Because there is an MMO based on the Warhammer universe.
As for open-world RPGs, I'm not sure anything can compete with an MMO because the worlds are ever changing/increasing. As far as MMOs, I really loved Dark Age of Camelot. Lots of different areas, lots of quests, lots of things to see and do... until they destroyed the game with their 2nd expansion. (As far as I know, it's still running, though it's down to a single server now.)
Mount & Blade is an extremely sandbox game, but not very "open" to exploration.
Gothic 2 was the closest thing I found to mimic the same type of open world feeling as TES. I never could get 1 or 3 to run so not sure about them, but Gothic 2 was ok. (Gameplay was a little harder then I think it needed to be, but whatever.)
And those of you saying Two Worlds is so great? Why? I couldn't honestly get past the clunky graphics and the absolutely horrible voice acting.