Adding a gunsmith toll (item) that can be combined with some guns and atachments would be nice, as an example, combining it with the m16 would remove the handle grip and add a picatiny rail on the top, combining with a hunting scope would increase its magnificence from 12x to 16x, or even combining it with a cobra optic would make a ak adaption to picatiny scopes.
Costumization is also a great idea to be readded in game, with sprays cans and knife marks on the gun stock.