I'm thinking that the next month's GWG game will be possibly one of the Far Cry games, most likely Far Cry 2. The problem with this is simply that the Far Cry series has very little to no DLC, with DLC being a distinctive feature of all of the other GWG games so far.
Oblivion might be something that would be released for free. It's a decent game, and most everyone has it (much like Halo 3, a previous GWG offer). It also has a plethora of interesting DLC (Shivering Isles, anyone?). And with the release of the new Elder Scrolls Online, Bethesda (or whoever is in charge of such things) would love to broaden its already-gaping fanbase.
We might see Modern Warfare 2 offered, but I'm not too sure. I would personally be happy if this were to happen.