One of the best features in Skyrim that's rarely given the praise it's due - and really needs to be taken on board by all other open-world games - is the fact that the game itself procedurally generates content and missions based on where you haven't been, to ensure you visit every inch of the world if you spend enough time there. In addition to this we'd love to see the world itself come to life a bit more with things like any warring factions doing battle at random times and particularly reputation-worthy creatures going about their own day/night cycles. Remember how great the various sieges in Skyrim were? Providing the after-effects are either somewhat preventable or not too severe, the idea of these sorts of things happening regardless of your input would increase the idea of a living world tenfold.
