For the Preds, a small kinife they can chuck that digs into the skin of the preyt and then explodes. I don't think the aliens need new powers (it cheapens them), but if I had to, venomous tails would be nice. Preds are better to give new weapons, because they have technology. A new weapon would be more realistic and practical. The Xenos, on the other hand, can only get a new power by some kinda phisical mutation, and then you have to explain how it happened, ect. Also, it would make it seem to much like some new gimick.