DotA (Defense of the Ancients) (PC/Mac Mod of Warcraft III)
While the game it's self is actually unsupported by Blizzard it's self, a community in which has devoted it's self and zillions (exaggeration) of hours to it would be rewarded greatly if this could reach a multiplayer base such as the XBL community. To think that a game mod which is not an official mod of a game made nearly six years ago is still one of the most discussed and played games is insane! DotA showed that when a community gets control of something it's lifespan can last so much longer than it was intended to. DotA is a custom Mod for Warcraft III , later was updated as the expansion pack Frozen Throne was released. DotA has become one of the most competitive game on the market, being played at Blizzcon and World Cyber Games.
Adding a game like this to a console market network would not only bring in a few more people who would have tended to stay away from the console market but also add more edge and "fight" to a network that doesn't exactly have my competitive games, while all the highly competitive games are retail games being bought and resold this would be a good way for the masses to take place in some of the action without having to spend 60 dollars for the same 10 multiplayer maps, and while the game itself is dated i couldn't think of another game that would would add a great multiplayer experience as a stand alone title to the network.
Plus with the amount of work being done on it consistently you would never really worry about more content coming, not to mention the install of it could be stripped down to below 100 megs easily.
While this is unlikely to happen seeing as how the game is a mod. One could only hope.
And for your veiwing pleasure: (make sure you watch the basshunter video!)