I had an idea for a new game type. It would roughly be called Scenario missions/maps. Instead of starting out with nothing and having to build and expand your base what if some of the base is already build but damaged or there is some kind of penalty.
Example#1: Your marine team starts at the docking back where your ship dropped you off. Your team needs to get to the partially destroyed base, while avoid getting slaughtered, to get structures powered up before an impending attack. Kharaa may also start at a location other than their base (at another docking bay with a sealed door perhaps) but won't be able to quickly get to the marines.
Example#2 Kharaa have almost completely infested the map except for a heavily defended marine base in the center. Unfortunately the kharaa hives(or some kinds structures) are gradually dieing due to some disease. So in this scenario if the kharaa don't make a push into the marine base they will eventually become weakened enough that the marines can wipe them out.
Example#3 Outdoor: By holding a key but hard to defend structure the marines can use some kind of high-powered artillery but when it is being used all structures must power down.
I didn't put a lot of thought into all of these but you get the idea. Play by different rules at the start of the game or by ones that are level specific. Granted these games could potentially be a lot shorter, they would also add a bit of variety to the same game but on a different map.