Actually, not having a single player is the reason a large percentage of people uninstall the game, based on what I've seen from the uninstall surveys. Improving the bots is the first step. I'm not sure how far it will go from there. I think a Quake3Arena or Unreal Tournament style single player is probably what I'd go for if I did one.
I also think it would be fun to do some kind of a horde mode where you fight larger and larger numbers of enemies in each wave. Could be great fun for coop, too, if you just want to play with 1 or 2 friends who might not have the same skill level as you.