Its funny, as kids we tried to develop our own Aliens RPG. We noted the difference between the alien in Alien 1 and 2, and felt that it would be cool if the DNA of the host could effect the Alien, so we started mixing up various alien-animal combos. Like aquatic Aliens, etc.
I think our reasoning was justified as I think the later movies did some of that too, but I can't remember.
It was pretty fun but we didn't go very far with it. We started developing various weapons that would be used to fight them and one suggestion was some kind of mace for hand-to-hand that contained some kind of base in the spikes to neutralize the acid blood lol.
We gave some thought on what would happen if they got loose on Earth and concluded Earth would be fucked.