You know, I'm reminded of an article I read shortly after GTAIV came out, which said that the game wasn't very good, with repetitive missions and unresponsive gameplay, but the city it built was nothing short of awesome. (It may have been on this very site. It may even have been an article by Mr. Young himself. My memory isn't very good, dur dur.) It put forth the idea that the developers created only the city, with other developers creating the games proper, allowing every kind of game to take place within it. Replace 'other developers' with 'modding community' and you'll have something very interesting. You'd essentially pay for a big hunk of nothing you could download endless free games for! (It's essentially a console, now that I think about it.)
My personal idea would be if there was, in adition to a city everyone built one, a canon story, with intertwining characters. Kind of like the old push by adventure game makers Reality-On-The-Norm [http://www.realityonthenorm.info/index.php], only instead of a series of adventure games it'd be a series of completely different games - a shooter here, stealth there, maybe a couple of racers as well.
Of course, it'd never work, because modders only work exhaustily for free on a game because they love it, and it's much harder to fall in love with mere potential. But I can dream.