An "I want to play..." setting
Basically, allow me to set my status as willing to play specific multiplayer games over XBL, which could then be searched by people on my friends list. An example:
Say I'm playing EDF and I also own both Catan and R6. Now, I'm having a good time playing EDF, but I'd have a better time playing Catan or R6 with some of my friends, and some of them are logged into live while I'm playing EDF. However, like most of the people on my friends list, these are basically fellow gamers who I really don't talk to outside of XBL, so I have no idea if they still like playing Catan or R6, or even if they have those games.
BUT, if I could set my online status to "I want to play Catan; R6 - Coop", then a freind who wanted to play Catan could do a "search for interested friends" search and get my name (and a few others), with the benefit being that the odds of me accepting a game invite would be pretty high.
This would also help the longevity of online titles and increase the purchase of additional content; let's face it, most games have a pretty dead online community after 6 months, even among friends, simply because people sell games or have moved on to different new games with (at the moment) large player bases. Most of the current FPS-style online communities are going to be decimated when Halo 3 comes out. But maybe I still want to play Shadowrun or Crackdown or even Chromehounds? With the "I want to play..." setting, I can keep older games far more active down the road, and will be much more willing to pay for additional maps/content since I know that the odds of people playing with me will be much greater.
Ideally this would tie into being able to edit my games list, too. Hell, just give me a "I currently have these games" list.