Increase to two strap slots
Currently, you can only have one "strap item" equipped on your back; a rifle, a crowbar, or a bow. I feel this should be increased to two, as I don't see what'd stop you from using a shoulder individually, or layering the items. I can understand putting a limit at three, since that'd be a lot of load and it would make retrieving the first item a hassle.
It'd also be possible to make the strap slot into a boxed slot, so you can't have a rifle and a bow, but you could fit a rifle and a crowbar, or a bow and a quiver (if that makes it in).