certainly W-Y in their infinite hubris would engineer various crimes against humanity and the holy human form so perhaps we should increase marine diversity by letting people choose their species with a variety of choices
options include:
cat people (upper body - cat, lower body - human)
dog people (actually they are just dogs, no people parts involved, but they do have integrated cannons though)
lizard (xenos with opposable thumbs on the side of the marines)
the toiletmen (no further explaination needed)
the nikondi cameras
regular human (boring)
upp
vampires (twilight variety due to balance concerns)
gnomes (S+ tier)
elves (20% taller for humans so they are more easily hittable)
Xenos with opposable thumbs exist in the game. It was some surreal shit watching a Drone pick up a M41A, walk into the Medbay and kill every single Marine and doctor it could find.