A few ideas now that we're in Source
1) If an onos jumps from a height, he'll crush any marines he lands on. I always found it weird how this 2 ton space cow could land on a marine's head and leave no mark.
2) Ragdolls. You KNOW this is sweet.
3) Revamped charge. The antlion guards come to mind. But make it physics based. If you can sandwich a marine between you and a wall, he's a pancake.
4) Knockback to use the environment. First of all, add a disorienting effect to compensate for how you just gave the marine more distance. Uses? Knock marines off cliffs, into electrical wires, into furnaces. Or sharp sections of the wall?
Just some ideas, take them or leave them. And leave your own please.
Thanks fatpower for the sig!