If anything, removing minimum damage could be overpowered; with tough as nails any rapid fire enemy (ie. arachnotrons) would be utterly harmless, and berserk would effectively become invulnerability. A few more quick ideas, though ...
Cockroach
Requires: Ironman (2)
They say cockroaches could survive the Apocalypse, but you'll still be there to squash the sons of bitches underfoot. Each level of this trait grants you immunity to acid, lava, and thermonuclear explosions respectively.
Unstoppable
Requires: Ironman (3), Dodgemaster (1)
Blocks: Eagle Eye, Finesse
You're the Juggernaut, bitch! And demons better stay out of your way, because each time you run up to them you cause immediate melee damage and knock-back.
(NB Pushes enemies to the side first if possible, straight back otherwise. Only triggers when you weren't next to them before moving.)
Hulk
Requires: Ironman (3), Whizkid (1)
Blocks: Hellrunner, Son of a gun
You think that piddly little green armor is really enough to cover your overgrown ass? Hell no. Now you can wear two sets of armor at once, and the benefits stack!
(NB Each set still takes full damage when attacked, so you'll burn through your backups quickly unless you're careful.)