As it stands right now, thorn as a stat is poor design.
All damage in the game is currently takes skills to land (insert joke about playing kitsu takes no skills here), so having a stat that rewards you when you fail does fit.
Every stat still need you to land something to reward you, whether it is so you can hit harder (Atk), more often (Atk spd/CD) or more access (Spd/HP to survive approach). Thorn remains the one stat that rewards you for failing (taking damage), leaving no counterplay for the enemy since the only way to beat it is to not hit you, but not hitting you means they can't win.
What I pitch is for thorns to convert thorns into Rage fire or Rage Burst:
Every time you take damage from a character, you gain a rage stack up to 5 stacks.
Rage fire causes a fire burst around you every time you are hit, based on the amount of stacks. This rewards the high survivability of tanks and the longer they stick around an enemy, the more damage they do. This gives counterplay by rewarding those who properly keep distance or spaces out their attacks via hit and run.
Rage burst would do bonus damage on your next skill hit based on the amount of stacks. This allows you to feel like getting hit on the way towards an enemy is only charging you up for your engage. Counterplay is even simpler since all enemies have to do is dodge skills from the tank (which is what players are doing anyways).
This also weeds out the item for assassins unless they build tank as well since they wouldn't be able to take 5 hits and still engage unlike tanks with their high base health. Assassins don't stick around long enough to benefit from rage fire and goes for multiple skill hits quickly so they can't space out their skills for rage burst.
Visually, you can have the character get more red and have more 💢 on them the more stacks they have, letting everyone know the threat is growing.
There's probably more that rage could be used for, but for now, these 2 are decent ways forward.
