The point of thornmail was to make it impossible for to drain tank someone stacking defenses by building offense and lifesteal.
So here is another way to accomplish the same thing without upsetting either side:
Every time you auto attack the thornmail wearer, you gain 1 thorn.
Thorns siphon healing from the target and send it to the thornmail wearer.
10% per thorn, stacking up to 5 times.
A person afflicted by thorns will lose 1 thorn everytime they auto attack a target other than the thornmail wearer from which the thorn originated. (Hurricane bolts do not cause you to lose a thorn)
If the person hasn't been in combat for 5 seconds, all the thorns will drop off.