The aliens should be a little more fast and have only a 2 second delay time when getting ready to attack again.
Here are some suggestions for making the aliens more badass:
The bullsquid in Sven Co-op was damn fast. It spits at you and then it starts running at you while spitting and, when it reaches you it does a melee attack (tail attack and mouth push).
The Garg in Sven Co-op would pick you up and eat you the moment you crouched under it.
The Alien Controller in Sven Co-op had a orange ball attack guiding system, where that orange ball would follow you a bit and give up if it didn’t hit you in 5 seconds.
The headcrab would turn around more quicker to face you if it missed you (2 seconds for it to do this).