Yes I know the front page is an april fools joke but the rpg part actually wouldn't be too bad. For example, It would be really good for customizing aliens if all the upgrades were individual and not dependant on what stuctures were build. This way you could have one alien with cloaking and another with carapace but not both at the start game, instead of everyone using the same thing. Marines could have something similar like heavy weapons specialist, light weapons specialist, repairer, builder etc that added small bonuses to aim, reload rate, build time etc.
It would probably be hard to balance but overall it could be cool. Anyway i'm not trying to submit an idea here, just wanted to discuss possibe rpg elements.