I'm tempted to say Vampire: The Masquerade. Though not strictly an open world like TES and fallout games, it has open hubs which allows you to wander around at will doing side missions as well as the main ones. Because you can play as many different kinds of vampires, all of which provide a unique experience, the replay value adds up to many more hours than the game initially appears.
It's one of the few games that give you that fleshed out world you get with TES and Fallout. And like Beth games, it's full of bugs! Yeah! So if you do decide to give it a go, check out the unofficial patch (You'll find it linked in the Steam forums for that game).
It's in my top ten best games ever made ever which I'm sure means a great deal to you, not!