Everyone knows certain AP champions / casters are OP, so instead of nerfing, why not create a sort of " reflective armor " that returns AP dmg back to the caster / champion as well as spell dmg eg; ignite, etc. Create an armor similar to thornmail but returns a % of AP dmg / spell dmg back onto the caster / champion.
** GET REKT**
Not a great idea, because a percentage of an ability will do noticeable 'reflection burst' to the attacker, and most abilities are AoE or multi-hit and therefore will punish mages for doing literally anything if someone with this effect is present (i.e. teamfights, where mages are most significant), as opposed to 
simply negating for one target for one spell.
Thornmail doesn't do this because autos are single target and (unless taunted) it will punish the mistake of attacking that person with Thornmail and still retains a decision point.
This new item could be called "Ancient Hawthorn Tunic" (I picked this name because the legendary Merlin the wizard was supposedly captured by the Lady of the Lake via a hawthorn bush; so this would be perfect to reflect the use of the item; also the secondary reason for this name is because a cloak [Negatron] and a "mantle"/belt [Null-magic] when combined could, in theory, make—or at least be a fair trade for—a tunic [Ancient Hawthorn])!
Statistics:
Abyssal scepter essentially does this. It boosts your champ's ap damage (also reduces enemy MR) while blocking incoming magic damage.
Make reflect half of the % of thornmail. Item cannot be stacked.
In writing this i understood that there were other items that negated dmg. The whole idea was to create an item SIMILAR to thornmail, with that in mind we will call it " MIRRORMAIL " or " ANCIENT HAWTHORN TUNIC " like Osearic has asked, lol.
I believe with an item like this it will make AP champs more strategic in the use of their abilities instead of just spamming away and melting an entire team eg: eve, donger, vlad, vel koz etc etc etc. An item like this will add a level of danger to them especially vs. a melee champ they can just sit back, cast and run or kite. Also, champs get nerfed... why ? dmg is too great, so, compensate on buffing preexisting items or creating new items that are upgradeable to different tiers. Obviously there are pros and cons to it, but thats for the great people of RIOT to do the number crunching and for the pros to test and give appropriate feedback.
We want carnage, mass killing and mayhem... lets add some danger to it.
and oh yeah....
GET REKT
