Ask Gilbert Arenas.
So what aspects of the guns can we modify again?
barrel curvature ? folding stock ? bipod ? MK ? camo ?[yep, including “badass gun” words on forearm;]
folding stock would be nice, good for tight ares, but with no prone, no need for bipod
no prone ? :[
oh yeah, relatively small areas mean nearly pointless to prone even if it had ingame.
then bayonet maybe ? or single-shot underbarrel flamer[unlockable only for engy] ?
flamethrower attachments would be completely awesome… highly unlikely at this point… but most definitely awesome.
because flamethrowers make everything better. no exceptions.
yeah, or soap in socks 
"just gimme wrench and gimme the enemy"© btw replace knife for engy for something more appropriate, finally 
Ooh. Non-lethal weaponry! Doc Heller would be proud.
Why not a blame launcher?

I don’t know if these spots are for every gun, or if some guns have different spots to customize. Comparing a minigun to an AR, you could see where I would think that they might have different spots for customization.
I was just listing some examples off the top of my head, really. i figured other peeps could fill in the blanks.
Edit: The post is now fixed.
actually more care about ammo variety.
for example in real-world distance between 0.223 M885 and M995 &similar 5.45х39 shift from 7Н10 to 7Н24 pack lot of difference, both in precision, AP rating, splash/recoil and etc.
Ammo variety is a luxury of the modern world. On the Ark, ammo is a bit different.