I know what you mean - it may (maybe not) be worth your while looking at Fallout 3. If you play it as a linear game - go there, do this, repeat - you'll finish it in two days. Play it as I am, as in an exploration and scavenging game, and it's sheer scale will astound you. I've been at it since the start of November, and still have a long long way to go. It's about as un-linear as you can possibly get, with the exception of EVE Online - that's just ridiculous!
Fallout 3 is "essentially" (in all the good and none of the bad ways) Oblivion with guns - if you want never ending awesome gameplay, and don't mind some sword and spell action, Oblivion is one of the best there is. The mods out there are just breathtaking, and make the game so much more than it could have ever been from a studio - just superb. Fallout 3 is getting there with the mods, but it'll take a while more.
Just some more food for thought.