I was reading some interveiws with the skyrim dev team and i saw that alot of fans had requested co-op as a feature.
They went on to say they started to work on it but stopped because they didnt want to split the team, resouces and what-not. Which is a perfectly valid reason i wont hold that against them.
My idea is why not produce a Co-op DLC or bundle Co-op with an expansion, the team would of finished the game and would be split up anyway (bug fixing and DLC). if worried about the 'narrative' they could gut the main mission out and leave all the other quests (even if you had to use a seperate character)
Red dead proved just wandering the world in with little to do in Co-op was mighty fun, just imagine wandering around skyrim taking on missions and discovering stuff with you co-op buddy.
Also i'd like a hardcore Co-op mode, not only share the funuctions of fallout new vegas' hardcore mode but if you die you have to make a new character, that would promote co-op team work
so.... Would you pay extra for Co-op in an extremly polished Single player game