or they could make it as an add on type thing and the people that want it could download it and the others could if they wanted too or leave it be... i say that the ones that dont want it dont have to get it, but they shouldnt make a difference on the ones that do, they wouldnt have to buy the disk or download the content if they didnt want co-op
First there are technical problems of creating such game - let's see how Mass Effect 3 does game with hard focus on character-based plot, and adding multiplayer to it - will it be a tacky deathmatch like Uncharted 2 and Bioshock 2 had?
Creating not only technology, but also content for two players is a great production risk, and we already know that TES games tends to have corners cut to release the game - because they are ever evolving (just cause modders add stuff doesn't mean the original programmers don't do nothing). Just by adding one player the world should be scaled and new content made so it wouldn't feel like a gimmick that the modders are working right now - not to mention if the number of players would rise.
A solution on CO-OP would be creating in game place for players to meet in between games - like the current mods try to do all over the game world. It would be closed city arena (you'd be logging in to rooms when entering) where players wouldn't be able to save the game (to prevent saving when players kill each other) and since it's not diablo they wouldn't be able to drop items. It could add player vs player content - like an actual arena. Players would have to relay on hotkeys, and using potions and changing weapon would always happen in real time. The winner could gain some fame, gold or even the loser's weapon.
Next game wouldn't need to take place in Imperial City to create such arena - it could be "traveling" arena kind a place or imperials would be starting the arena in all over timriel - nice set up for yet further game and call back to the first TES.