I was thinking, while watching the making of special on Alien
Resurrection, of Ideas for an Alien/??? X-Over:
1. Teknoman/aliens: This is obviously Warhammer's territory, but since
he's probably busy working on his tekno/eva Xover, and (I think) an Xover
to tie all the other fanfics of his up, I figure he's got his hands full,
which is sad since he's the best SF fanfiction  writer I know. Anyway, I
think that the alien's acid spit/blood/pee would probably sting the
teknoman and cause mild pain, but not do any real damage.
2. ranma/aliens: Howzabout a hive in the Tendo Dojo? Or an alternate
universe fanfic. or ranma gets recruited by the Marines. Or- well, I'm
starting to see why there's so many ranmafics out there.
3. Sailormoon/aliens: Yeah!!! Kill!!!!! No wait, that's my RPM
series...^_^
4. eva/aliens: The angels use their last resort; the most feared species
in the galaxy, and set a queen loose in NERV.
And the list goes on and on....... not to mention Predator. In fact,
Ranma vs. Predator would be awesome, but that's a whole other post.