Talking about a back slot: If you can opt to not take a backpack, that should also be viable. What always bugged me that it made no sense to not stuff your soldiers to the brim of their carrying capability, i.e. in X1 there is a malus for carrying too much but no bonus for carrying less. Stalker had this as a very neat system of increasing encumbrance, which means you could opt to go in very quiet, fast and light or as a heavy, slow walking ammo and gun locker.
Using a similar system it could be really viable to do light pistol scouts with increased TU (so they can move farther and use more items and stuff) and reflexes (so they don't get shot at as much. Extra cool would be if you could drop your backpack to get these bonuses temporarily. On the other end you might have heavy soldiers who cannot even do certain things, akin to how (at least in X-Division) the Predator wearers cannot use medkits.
As another suggestion on equipment, how about a backpack that provides a smoke screen when (while) being shot at?