The Planescape universe. It would be a very daunting task, but seeing all of the alignment worlds fully realized with Bethesda's open-ended design would be a dream come true. Frankly, though, I don't think they're capable of it as they are now.
Take that as an open challenge, damn you..If I say that I want an MLP open world rpg, am I a bad person?
Yes you are, and if Beth
ever did such a thing, I would hate them more than any other gaming company.