The Xenomorph evolution

That was O'Bannon's original idea (the evolving bit; not using body parts and fluids as weapons).

It doesn't seem to be born out in the films however.

Yeah, I don't think your basic Xeno has that ability. However, if it's true that you get Xeno-variants based on the type of host creature they use, then maybe a host creature with some spontaneous evolutionary capability could produce a Xeno with the same potential.

Imagine one that could communicate!