Thornmail for Magic Damage?

I feel there needs to be a Magic Damage equivalent to Thornmail (Thornbarrier?) that scales with MR and deals physical damage. Riot can set it up where players can only buy 1 of them in a match just like they have for various other items.

A champion like Rammus whose main uses are his CC and reflection damage for trades suffers to hard when forced against an AP comp that don't care at all about reflection damage (as they have low auto attack damage and build AP the Rammus/Thornmail wielder wouldn't have to much armor so the damage won't be scaled to start with, making Rammus' W and Thornmail's reflection damage do nothing). It also cannot be denied that Magic Damage is the dominant of the common damage types and has been for quite some time. This might help mitigate that some.

Here are some changes I have for Thornmail and the "Thornbarrier" concept : Thornmail - 100 Armor UNIQUE: Reflects 20% (up from 15%) of the physical damage taken (+ 25% bonus armor) from basic attacks, on-hit effects, and non-ranged physical damage spells back to its source as magic damage (Calculated before any resistances or reductions are taken into account). [Non-ranged physical damage spells meaning things like Talon's new Q where he lunges/shanks and Darius/Garen's spins. Ranged would be pretty much anything blockable by a wind-wall or traps.)

Thornbarrier - 100 Magic Resist Builds from : Null-Magic Mantle + Negatron Cloak + 1180 gold (total 2350) UNIQUE: Reflects 20% (15% for AOE) of the magic damage taken (+ 25% magic resist) from magic damage spell casts back to its source as physical damage (Calculated before any resistances or reductions are taken into account). [Like Thornmail, it wouldn't work on DOT effects such as poison or bleed based effects, only from the initial spell damage proct.]