Well, thanks to the modding community, Oblivion has a co op feature. Whos says this wont happen for Skyrim on PC. But Xbox, not happening.
Oblivion has no multiplayer whatsoever. All of the multiplayer mods died. Some of them managed to get pretty far, but they
all died. It was far to difficult to make work in a bug free manner and a lot of things just outright didn't work.
As far as who says future TES games won't have multiplayer? (It's funny because I'd think that you being so on top of info would know) Todd Howard himself said it's not going to happen and they have no plans to make it happen. It's something they really
don't like the idea of doing in their games.
Save the multiplayer for the inevitable Elder Scrolls MMO. All multiplayer would do is take developing time away from making what makes The Elder Scrolls TES. Call me old-fashioned, but I'd prefer a fully resourced single-player experience.