The only way I could see this REALLY working and getting a good amount of users on would be building the feature into the OS to run on bootup. That way you don't have the lazy people who don't want to bother installing it and if they really want to they could disable it. In the beginning I would jsut imagine something simple like letting users create a username and password on the first start up. Then the systen would be tied into the OS so you could have alert boxes (drawn on top of everything) like Live and PSN have. Add simple messaging, P2P file transfer, freinds list, voice chat, etc. Then based on how many people use this we can determine if a lobby system is needed. But I will tell you this, you could get some programmers lashing out at you by making them "conform" to your system and programming for it. This will be the biggest problem, getting everyone to agree on a standard. Good luck with that if you can get things that far, personally I'd jsut be fine with this upgradable community network for right now ;]
That is in the WAAAY-T.H.B. try thinking Xfire. its a pc app. its the more likely answer that gets the job done.. just mix it with irc.. and you get the idea.