Quotepurebreed aliens are the same no matter the host according to earlier games...
And yet, in Alien3, the Xenomorph is different, because it's born from a dog.
Also, what of the predalien?
Perhaps I misunderstand. What do you mean by "purebreed"? By it's nature, the xenomorph needs a host of a different species.
As for a sequel, if for nothing but gameplay experience, I would like to see a split-screen option. Obviously for offline multiplayer but also perhaps for a co-op campaign option. This could easily be incorporated, storywise, with the non-humans. The Yautja is not reffered to by a title anyway, objectives directed at "you" could easily imply plural. And the xenomorphs are hive creatures, very conceivable that there would be two as as easily as one receiving the same calling from a Queen. Clever wording could leave the story open to this option. Not an essential, but always a welcome option for playing a game's story mode. However, adding this option would prove the most... challenging for the human campaign. Ideas?
Also, a random thought.
Though I prefer to play as Yautja, I think it would be fun to add playable power loaders (customized, and better armored, of course) and APCs (the armored personnel carrier from Aliens), for the humans.
The power loaders would deal high-damage "jabs" (impale then "spread claws" to kill) for light attack and a slower, but tracking grab for heavy attack. Great for killing, but a large target for a plasma caster, or dodging, fast attack xenomorph.
The APCs are good for getting through hordes (such as an alien homeworld hive), and can run over anything in front of it, while keeping claws out of flesh, but enough damage to it's sides can expose it's "chewy marine center". Easier when it's not moving, of course. But keep in mind while you don't want to be in front of it, xenos' can stick to anything. Driving, you may have to scrape a wall..
Speaking of xenos', an equivalent for them could be on certain hive stages. After certain conditions have been met (wall broken down, enough kills, set time past), being the first xeno to press the action button next to the Queen lets you become her. Fortunately, her massive size limits her to larger areas, keeping certain tunnels and corridors safe from her wrath. Of course, the power loader may help against her. And it's grab, when initiated against a Queen, would grab with both claws. Perhaps both sides would be prompted to tap the jump button repeatedly, to detemine whether the Queen escapes the grasp, or is thrown. Both outcomes dealing damage to the loser.
Again, I prefer Yautja, but I would be fine with these additions with no predator equivalent.
Just some quick thoughts...