Why not put a unique passif on thornmail since now it will be probably shit (200 more gold for a worse passif than before :D) Why not put it a nice Unique passive one it something like this:
Unique passive: Whene hit by a bassic atk inflicts Grievous Wounds to the attacker's for 2 sec. (Sure you will need to nerf or buff thing on it but it would be fun)
It would fallow the Idea to make 
situational.
SORRY FOR THE BAD ENGLISH
Because that'd be broken. You'd see vayne go 100% pick or banned simply because she would be the only adc that could actually kill the tanks that build this. Most adcs mid late game can only hit what is in front of them, and if what's in front of them is a tank that can negate their healing entirely while still dealing huge chunks of their health just from TM, it'd be broken beyond belief.
i Don't Think Thornmail is necessarily OP Right now, but It Is Guaranteed to be in a League of Legends EndGame Tank Build. the Very fact That League Of Iegends Tanks buy Thornmail every game means That it is Too Good An Item. perhaps Other League Tank Items are deserving of A buff. i don't Know, Tank Armor Magic Resist people mains should be asked First
Lifesteal is an awful way to counter Thornmail. As of now, if you have Thornmail and the average 80 armor at level 18 and they only have the basic 30 magic resist, they would need 60% lifesteal to just break even. Armor Pen helps, but lifesteal does not counter TM.
The new Thornmail doesn't work out as cleanly, since it scales with your other items. But an autoattack for 1000, assuming that you just have TM and, say, the new Frozen Heart (270 armor total), requires 56% lifesteal to break even. It's technically a nerf, but lifesteal is still not the way to counter Thornmail.
It already is a good item vs life steal.
It already reduces the effective healing from lifesteal since you hit someone with Thornmail, you take damage as well. Grievous Wounds on Thornmail reflect would make it insanely broken and completely invalidate almost (if not all) adc/marksman from actually being a pick at all.
Seems like they want to phase out Grievous Wounds anyways. It's hard to balance around: champions that warrant getting GW either do extremely well not facing it or fall apart when they do. ADCs have one of the more healthy sustain mechanics since they have to actively put themselves at risk to heal, and if positioned properly, will get CC/blown up/die and lifesteal CANNOT save you if you aren't attacking. Other champions that would be more important to take GW against (Swain, Mundo, Volibear, Warwick, heavy sustainers that can be moderately tanky) have balance issues revolving around it, either doing super well when the enemy has no Grievous wounds or doing poorly when they do.
Thornmail technically already counters lifesteal by virtue of reducing the damage you take and therefore the heal. i think this should be a separate item all together or perhaps a mastery?
