A fond dream of many gamers is the crossover game. That's one of the reasons SNK lasted as long as it did. Its one of the reasons Kingdom Hearts sold gangbusters. Its one of the reasons people write bad fanfiction (and good fanfiction, too).
A fond dream of some of the slightly more inspired gamers is a massive "cross pollination" of games, where you can link games (or game data) to other games regardless of the original source.
For example, you could have two people with two DS. One is playing Metroid, the other is playing Ouendan. Samus' actions reflect in the play of Ouendan, and the performance of the cheerleaders affect Samus' gameplay. It could be supportive - Samus gains bonuses if cheered on well - or antagonistic - the enemies become faster and more numerous if cheered on.
The basic idea being that, in an insane and perfect world, any number of games could be combined into a bizarre meta-verse. Save your character from Final Fantasy Online, port him over to a table tennis game. Or Phoenix Wright. Or whatever.
Of course, the idea is silly. The overhead would be enormous, if it were even possible at all.
XML was based on the same idea, except without any of the awesomeness. "Format-free content": interpret it in whatever way makes you happy.
This is the same idea, except that every game would need an interpreter, and that would be a rather complex piece of machinery. Add on balancing systems and content restrictions, you end up with something as complex as, say, every piece of software on your computer combined.
But... if there was a baseline...
Something which manages content. And by "content" I include gameplay and levels and scripts and so on. A piece of brilliant middleware which helps you build a game - you supply the engine, you connect it to the middleware. Then you build the content, and the middleware plugs it into the engine.
Or, you know, you just plug in random crap from existing games.
No, of course it wouldn't work. Our data processing capabilities simply aren't that good.