Multiplayer.
I don't mean competitive FPS gaming like the Halo series, I mean having anywhere from two to four players immersed in the same story, but playing their own part. I keep telling myself, all I would need to have the perfect role playing game, would be to have an Oblivion with multiplayer. Much in the fashion of many novels and games, you could be one of a few travellers who have fought in their own way for the same cause. The possibilities become endless with the introduction of this 2-4 player co-op, and how you interact with the world. One person could make camp while others hunt for food. A sneaky rogue could sneak behind the defenses of a fortress and open the gates for his fellow men to break through. There are countless of scenarios, but simply adding that opportunity to share all the features of singleplayer, with another player, whether it is just one, or more, I think would seriously make Elder Scrolls the PERFECT game.