Don't know how hard it would be, but I think it'd be pretty neat if each character had multiple clothing accessories that you can either pick or are random (like empire: total war). They wouldn't have to be big differences, different scratches on armor, different numbers on uniforms, but I think it'd be pretty cool :lol:
Sounds like hats in TF2 (only not actually hats since that would be a little too similar to TF2). I don't think it's too important, but it could be interesting. I wonder if the customization features that have already been mentioned would have a similar effect...
Changing the face of you character would be good, So every one didn't look like the same guy. It looks like you can already change pieces of your character depending on what perks you have on like the ammo mule and stuff.