With backpacks becoming smaller, a neat feature is to have weapon slots on backpacks. An ideal situation should have some backpacks should have only one weapon slot while some may have two weapon slots, and some weapon slots being only for melee [not knives], firearm, or for any weapon.

Like here is an example:
Slingbag, 1 melee slot
Alice bag; 2 any weapon slots
Tactical Bag; 1 melee slot, 1 any weapon slot
Mountain Backpack; 1 melee slot, 1 firearm slot

You could also restrict the size of a weapon from being attached to the bag

Combat Backpack: 1 melee slot, 1 Firearm slot [21 slot limit]