Two Thornmail rework ideas.

Thornmail currently feels extremely underwhelming on about 95% of the roster with the few champions that can use it effectively find it underwhelming as well( ) Their absurd armor steroids make the item semi viable on them.

The first rework:

Thornmail, 70 armor, 2400 gold cost, and an active.

Active: When activated for the next 6 seconds thorns come out and reflect 10%(+10% per 70 bonus armor, capping at 50% reflected damage which needs 280 bonus armor) physical damage to the attacker(does not reflect some physical damage dots such as Talon Q). The damage reflected is dealt as physical damage and the reflected damage is not reduced, just the enemy takes that damage as well based on your armor. 60 second cooldown.

The second rework:

Thornmail, 100 armor, 2400 gold cost.

Passive: Thornmail reflects 15%(+10% every 70 bonus armor excluding Thornmail) as magic damage. Inflicts grievous wounds on the target if they attack you when below 30% health for 4 seconds.