the only 2 items available for the inventory slot are medic bags and tool boxes so unless you plan on loading up on 5 medic bags there is almost never any reason to use a backpack.
My suggestion for for this is to add different kinds of ammo cans that you can deploy on the field but with the caveat that each can only resupply a specific type of ammo are heavy and can only be used 3 times per can until its exhausted. An example would be an ammo can for regular ammo, grenades, and AT weapon ammo.
Another thought would be the ability to carry extra building materials as an engineer.