One of the things that separate MS 360 console from the PS3 when it first came out was the Gamerscore system. This Gamerscore became an addiction for a lot of gamers and achievements was the engine that drove it. Gamerscore is at the point that people who want to move to another system are hard press to give up that number they worked so hard to achieve. Gamerscore even made Sony introduce their Trophy system in response.
The next addiction that I can see happening if MS so choose to go this route is to make the Drivetar system within Forza global to the console. Here is my reasoning on why this could be just as addictive to gamers as the Gamerscore Achievement system
The feature of the DrivAtar that I am talking about is how it goes out and play within other people singleplayer or maybe even a mix of singleplayer/multiplayer games. Your Drivatar is always learning about how you play within the game, how you round corners, how you pass opponents, what tricks you use and how well you manage the course. There is a skill level giving to your Drivatar and it goes out and get downloaded to other gamers system and become their opponents. You are notified when your Drivetar win games and are giving points and awards for when it does.
Now picture this same system setup for your Avatar that pretty much does nothing on the 360. So instead of just dressing up your Avatar MS could set it up so that your Avatar can be download as a AI character within multiplayer or even Singleplayer games.
Think of a multiplayer game that has a singleplayer component which you can practice in. I know a lot of people do not like playing MP online but they would not mind playing a MP style game with bots in them. Based on skill level, people could download an AI Avatar based on your playing style and play a perfectly balance match that way. If your Avatar is downloaded it can gain points by doing the things you normally do in a match. These points and awards would be sent to your Avatar and displayed there for your profile. If you really want to get crazy you can have the Avatars ranked between friends and global leaderboards.
A system like this can easily be setup within games like COD, Battlefield, Crysis to just name a few. So now instead of playing against the prescripted AI characters within a MP or SP game, people can now play against other people AI Avatars. This would be a great system for developers to put into place because people would be interested in advancing their Avatars abilities thus they would play more games and increase the lifespan of that game. It would also add sort of a meta game within the MS Ecosystem where not your Avatar actually does something and can be shone off like a Gamerscore or achievement. I believe this can make your Avatar feel like an extension of yourself, going out there kicking butt and taking names.
Of course this system is not limited to MS, its just that MS is already develop a system so they probably would have first jump.