Just make Thornmail an active

it's not hard and it solves nearly everything wrong with the item

passively disincentivizes attacking? active means you have windows to play around and can bait it

Too hard to balance passive return damage? active means you can make it appropriately strong when it goes off and balance it around having a cooldown

remove the armor scaling while you're at it; no more hard countering it with black cleaver, it's just raw return damage now.

let me wreck marksman scum again, RIOT