A
working multiplayer mod was
indeed done for Oblivion. And Skyrim is most likely multiplayer-moddable too.
But it took several years for the modders to develop their knowledge of the game and "state of the art" modding sophisticated enough to begin thinking about multiplayer.
Although MP TES would be epic (coop... or even MMO-like with a small number of players), it would have required an awful amount of time from the devs. Furthermore, they aren't used to it at all and could have fucked up everything. I'm already happy with a good solo game.
Just like with Oblivion, the vanilla version will be good, but show it's limits after a few hundred hours. By that time maybe a few good leveling, rebalancing, feature-adding, cosmetic, content-adding, etc. mods will be out and players will be able to restart the game from scratch and play something different and better
Iirc there's no "M&B-like combat" mod for Oblivion. I think it simply wasn't possible due to some hardcoded mechanics.