DoD:S would be an awesome game to add.. I quite enjoy it, though it's probably not for everyone.
http://store.steampowered.com/app/300/ $10, though I got it while it was on sale earlier this year for ~$2.50
The best way to get something like COD4 added would be to check on the Steam group to see how many people already have the game. Make a post on the Rsbandb Steam page and see if anyone will respond. I doubt that people will actively check the end of the thread to put their info for the game in, but I would ask that anyone who has the game and would like to have that on rotation to please leave a comment.
That being said, though it would be a good game to play (after all, it is the non-nerfed Modern Warfare game), it would be the most expensive game have ever debated playing. A new PC version (I don't know if I'd trust a used version, as the CD key could be banned from online games for various reasons) is $30 at any online store.
This all being clear, when we look at other games that we offer for voting, the cheapest games tend to win. While it isn't always the case, it seems as though we get more people to attend if the barrier to entry is lowest.