Hi, I beleive that an additional equip slot (backpack) would make a good addition to DoomRL.
Currently we have:
Weapon
Armor
Boots.
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In Doom I and II the marine could find backpacks that doubled his ammunition capacity, I think that there should be two main archtypes of backpacks (explained soon) with subtypes.
Type A - Ammunition
A.1 - Backpack. Adds +20% onto the maximum ammo you can store in any given pile in your inventory
A.2 - Same as the above, Except +50% bonus and -5% speed.
A.3 Double your capacity, -10% speed.
Of course there is endless expansion oppertunitys and one may choose to opt for a specialised backpack (one ammo type)
Type B - Weapon Loaders
Theese are practically an extension of your guns clip, Except you wear it.
B.1 - Rocket Pack - Gives say, 4 or 9 extra clip space in a rocket launcher when equipped.
B.2 Energy Pack - Plasma rifle space is improved by 50%
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As you can see there are many ways, And balanced ones at that to work theese in the game.