I feel like thornmail is a toned down version of that pattern. Not saying it was hidden OP like how zzrot literally was, but thornmail is currently an overall very good item in my opinion, with little to no reason for it to be buffed.
alfavhunter1 votesi think the only issue with thornmail is it should lose the base damage and get a better armor ratio but its true damage
ADC has to many AD MR item options for it to be very effective when it deals magic damage
the grevouse would change helped sure, but 1 mr ad item and 1 lifesteal item and they are outhealing it easily without sacrificing much damage
example
thornmail deals 20% of the tanks bonus armor as true damage to the attacker (thornmail gives 75 armor so just thornmail for bonus armor would be 15 damage return, but late game it would catch up with having more than 255 armor (which isn't hard to get for a tank) but it would be true damage now as well so the actual break even point would be around 150 bonus armor due to MR no longer effecting it, which is 2 armor items for a tank like thornmail and 
(in team fight of 3+)
this keeps it more reasonable early game and lategame as it now forces 2 lifesteal items to survivie the tanks return damage properly, but leaves the ADC open to mages damage still, instead of countering thronmail also being the counter to the mage AND the tanks damage and thornmail (most tanks deal ap damage on their spells)
changing it to true damage and removing the base value makes it much more of a 'do you counter build hitting me, or my mage's damage and my spells damage
i think it would be healtier game design to make this change to thronmail and 
can remain as is because its magic damage return so even with just thornmails 15 true damage (from only bonus armor from thronmail) would be more effective than the 20 magic damage on the vest due to it being reduced by resistances
that is my 2 cents
