In my opinion, something that would be quite smashing would be a way of making humans and aliens a bit more maneuverable. For instance:
Humans
1. Be able to do something like a roll to avoid charges and pounces more easily.
2. Be able to jump on top of aliens, or onto their backs for a moment. (And then use pain saw or flamethrower, etc.)
Aliens
(I can't think of much for aliens, they're main hook is speed...)
1. Possibly a way for dragoons to be able to hold a human in its jaws if done right. You know, as if it was eating the poor guy.
2. Maybe some sort of way for dretches to have a web-like strand, for swinging across hallways and hanging there for a moment. Wall crawling is just a bit too difficult to control.
Plus, some ragdoll physics would be neat... You know, get charged by a tyrant and go flying.