I'll just put this here:
Armor mods?
Like, something that increases the protection slightly? Reduces damage for a single type of weapon (vs. bullets, vs. shot, vs. fireballs, vs. plasma, vs. acid, vs. lava). Something that increases the percentage above 100 (as silly as that sounds).
I've seen power armor mentioned before, so you could have a mod that increases skills like powered armor would, adding a bit to HtoH damage, or speed, or staying power.
Reactive armor, that destroys itself while taking full damage from a single hit.
Deflective armor, which adds a change in vector percenage to the to-hit roll, which will send bullets flying in random directions away from the player (great for being stuck in the middle of crowds).
Maybe this is straying a bit from the Doom forumla, but the weapon mods are fun if you can manage to get an advanced weapon...