AP THORNMAIL?

To quote JustMyBassCannon (apologies if i fucked up the name!)

"Alright, you want my top 4 reasons why no Magic Thornmail?

1. First and foremost, coding issues. Item damage tends to be coded the same as spell damage when it isn't attached to basic attacks. Long story short, what this means is that there's a very high chance that the code could bug out, and two Magic Thornmails could reflect each others' damages. So, if you had two mages pick up Magic Thornmail, and one casts one spell, the damage reflection would go back and forth a number of times depending on how high the damage return was. If they threw their whole rotation, it could crash the game with the feedback loop. They would have to specifically focus on making sure the code could never hold a feedback loop like this. Plus, thanks to spell damage and magic damage not having the same meaning. AD champions would be taking spell reflection damage as well.

2. AD is not similar to AP, ergo Armor is not similar to MR. With AD, your damage tends to scale over time, whereas AP damage tends to come in bursts that take about a full second to rotate. While an AP champion is capable of 2000-3000 damage in a single second, an AD champion can match that damage during the AP champ's rotation cooldown (usually about 5-8 seconds downtime). Thus, when you buy Armor, you're trying to extend your lifespan by a number of seconds. When you buy MR, you're trying to survive the rotation so you can retaliate.

3. AD has a lot more control over their damage output than AP. AD champions don't have auto attacks that apply in a large area of effect. AD champions don't have uncontrollable bounce attacks that apply on-hit effects. AD champions don't have skill shot type basic attacks that can be body-blocked by someone else.

4. AP champions don't have a sustain mechanic quite like AD champions. With AD, you have Lifesteal, which is consistently sustaining you through combat. Now, your first reaction might be to point out Spell Vamp, but the problem with Spell Vamp is that it's a lot less plentiful and much less useful to most mages. Lifesteal is commonplace, Spell Vamp is not (and everyone hated it when it was).

Bonus: What is your solution when someone picks up Thornmail and "Magic Thornmail"?"

no

Congrats rammus now builds both versions of thornmail and gets apenta by watching the other team kill themselves.

I'm not sure if that's a good idea as Thornmail deals with Damage over time while your idea deals with burst. After a while, If you are greatly affected by thornmail, you can get life steal ( and lots of it) to counter. Spell vamp is not viable for all AP mids (or wherever) and there is a lot of burst to go with it. After using one ability, you could lose a ton of health with little counterplay.

No.

Tanks would build both versions and become legit unkillable (Galio, Rammus with these? Holy shit)