If they are some damn good models, why would you want to change them? I remember ACM had a customization for xenos but that was for your own soldier in multiplayer.
So since you can't play as an alien what point would it serve?
Think about it:
If you did a mod, the only one who would see it is yourself.
You might be able to share the mod, but you'd have to have others download the modkit and application that implements it.
They could take the Modification route like how PHG has and add feature changes through a mod menu.
Ei, big heads, or change alien heights, tail size, make all of them stand upright like big chap.
I dont know, im spit balling here but that would be the easier less invasive route for Mod support.
Personally, I would like to see maybe a mod that changes all the alien classes to one class of horde like ALL soldier type aliens from the JC film. Or make them all exploding alien types. I dont know how fun that would be though.
In terms of cosmetics, again,FOX /disney might hold the reigns on that one because they dont want people to mess with the product in the wrong way.
