Thorns is a fun mechanic in D3 and I think it can solve some problems in League

First off, for those who don't know what Thorns is, it is a stat in Diablo 3 such that every time you are struck, the enemy is instantly attacked (regardless of range) by an attack with base damage equal to your Thorns stat (this is modified by % damage modifiers just like your other spells).

Second, my proposal as to how it would work in League:

Thorns (or whatever you want to call it) would be a base stat with a base value and a growth statistic. Tank items would give you a % increase in base thorns like health/mana regen items. Every time you are attacked by a basic attack or hit by a spell, the unit that attacked you takes damage equal to your Thorns stat. Thorns could be physical or magical. It might be better for it to be magical due to lower amounts of magic resist and because I want thorns to phase out % max/current health damage on spells. Tanks have an enormous amount of max health damage in order to keep them relevant without being too overbearing, but I think Thorns would serve a similar purpose without the odd AP Amumu, AP Sejuani, whatever cases AND for another reason I'll get to in a minute. Thorns have no range limit, and they are instance based. Vayne dealing damage damage to you gets blown up, and Anivia dealing a lot of aoe can be damaged as well. Not everyone would have a thorns stat. I think no ranged champions or mages ought to have thorns. I think bruisers and skirmishers (possibly even melee assassins) should have some base amount with either 0 or a minor scaling amount, and Tanks should have a hefty amount early on and a hefty amount scaling. For comparison, I'm thinking 10-25% of base attack at all levels would be what Thorns ought to start out with

Third, what does this solve?

1. Early game melee vs ranged - historically, every ranged champion has been cancer in top lane vs melee. Vayne, Quinn, Lulu, Season 5 Lissandra shudder. The mid/late meta is pretty much what designates if someone can do it, but generally speaking, ranged champions get nerfed for their Top lane dominance versus melee. Thorns allows some retaliation in melee versus ranged at range so that it is not free harass. This same reason is why certain champions who are technically melee can dominate top lane (Pantheon, Maokai)
2. Ambient damage in teamfights - while we don't want all ambient damage to be unavoidable, making a sizeable portion based on bodyblocking (what a tank does!) as well as giving some more reward for sticking in the aoe to hold people down would be beneficial.
3. Replacement of this Max health damage creep. Look, Riot, I KNOW what max health damage does and how its scaling helps tanks without making them overbearing, but it's fucking annoying to be building hp and then realize, "Oh shit, Zac. Fuck." You said it yourself; health is a better read of EHP than resistances and Max health damage encourages building resistances. You can make a UI element that grows pricklier the more Thorns you have. That would be more readable than armor and magic resistance
4. Budgeting for Thorns in items late game would discourage bruisers and skirmishers from building tank items instead of what you guys want: bruiser items. That curtails balance issues with items. At least some of them

Fourth, what problems would Thorns create?

1. Satisfaction in use - Thorns in D3 are fucking OP. You can walk around as a Crusader with a Thorns build and just let monsters kill themselves, even one shot. I do NOT want this power in League. Thorns is supposed to be noticeable early game for trading and then be relatively unnoticeable for the rest of the game even though they should be adding an extra 15-30% more damage to tanks in aoe situations. Less satisfaction late game means less fun
2. "THAT WAS FUCKING BULLSHIT" - yeah, get killed by thorns. What are ya gonna do? [slayer-jinx-unamused]
3. Discouraging of attacking - Thorns aren't supposed to be noticeable mid and late game even though it will be significant, but the noticability of thorns early game may discourage people from trading.

Finally, how would we go about solving the above problems?

1. UI elements an particles. Flashiness that's not too busy ought to make it more satisfying. Alternatively, we could put a 10 second cooldown on Thorns, allowing them to be even more impactful at a time and adjust from there.
2. No fix. It's just part and parcel of the mechanic.
3. This could be addressed by base Thorns, extra regen on health pots, extra base regen, changing thorns to physical, or even putting a mechanic that allows you to ignore thorns within a certain window, rewarding expert play

Some examples of base stats and growth for Thorns:

0 + 0/lvl 5 + 0.5/lvl 0 + 0/lvl 5 + 1/lvl 8 + 1.5/lvl 5 + 0.5/lvl 10 + 2/lvl

Some example of thorn items and additions:

+100% thorns +50% thorns +150% thorns Now increase thorns by 100% and increases base thorns by 15% of armor

Urchin Armor 300 gold

5 armor +25% thorns

starter

Cursed Mirror 3000 gold

+50 magic resistance +200% health regen +150% thorns Unique passive: Thorns deal 50% less damage in response to physical sources but 50% more damage to magical sources

tank item vs dps mages

Junglevine Buckler 1800 gold

+50 armor +400 hp +100% thorns unique passive: Thorn damage is now physical. Every instance of thorn damage reduces the unit's armor by 1 for 2 seconds, up to a max of 15+2*level

useful for helping your teammates

Sunfire Detonator 3000 gold

+500 health +60 armor +150% thorns Unique passive: Thorns do not retaliate against instances of damage but store stacks of "biding time" for 4 seconds up to 10 stacks Unique Active: Detonation - Deal damage to enemy units within 400 range for magical damage equal to 50% of your Thorns per stack of "Biding Time". 60 second cooldown.

Tanks can execute if necessary. Do keep in mind that this would suck out some of the power of current spells that have aoe dps or a detonation.