How will the backpack be implemented? I see several options.
a) An item which gives more inventory slots, this would be probably a problem to implement.
b) Like in Alphaman, or bags in Nethack: an item that, when (u)sed, gives additional storage where you can put extra stuff. "You use the backpack. Do you want to (p)ut or (t)ake things?" You can have only one of a kind. Initially should be filled with ammunition, as in Doom.
c) Double the maximum amount of ammunition that you can store in one slot. This is similar to DOOM.
d) A slot that can store ammunition (e.g. 300 bullets, 150 shells, 30 rockets, 120 cells, all in one slot, or maybe just 480 cells in one slot). You can have only one of a kind. Close to DOOM.
I think b) is the best, and probably the easiest to implement.
BTW I usually hate roguelike games with tiny limited inventory, but the DoomRL system is very good. (In the Angband inventory I am annoyed by the fact that I can carry 99 potions of one kind, but I cannot carry 30 different rings.)