Call me unimaginative, call me the nadir of TES, but I'd like to have the ability to go dungeon diving with a mate. Arguments over loot would be great; "fight you for it!"
Plus we could find new ways to troll unfortunate enemy NPCs. Dual Fus Ro Dah could be fun...

I agree that the single player experience should take the overriding priority, it would just be fun to do if it were possible.
Other than that: a combat arena please
