Since Thornmail does Magic damage the primary counter of Thornmail should be Magic Resistance
Thornmail new: Applies Grievous Wounds to attackers
When ADC Stack Lifesteal makes Thornmail useless. You might even completely ignore MR and still be OK
Thornmail
Reflects 15% of the physical damage taken (+ 25% bonus armor) from enemies basic attacks
Bloodthirster
20% lifesteal
Fondling Gems1 votesI actually like this idea a lot, perhaps cut the damage down to balance it but it really turns it from either "Why are you hitting yourself?" Or "Why do you still outheal my damaaaaaaage?" To something that consistently negates healing. My one issue is that it becomes very powerful in certain situations such as when the enemy team's only AP dmg is a Swain or Vlad buying armour still is very effective vs them too.
YSeT9iEzJK1 votesyes..yes...YES
It's just a no-brainer to find a reason not to add this
grevious wounds should've been on thornmail from the beginning
xBushx1 votesThis is a +1 post and would help this late game carry problem. However, another concern i have becomes League of tanks again. It is WAAAAY easier to buy defensive stats to counter offense because it costs more.
SwiftKitten881 votesi just posted this.. like 2 days ago...
SlashStriker1 votesi just posted this.. like 2 days ago...
well sorry mate. Seems we ve been thinking the same think its just i posted this much later. I did read that someone sayd that Imaqtpie counters thornmail with lifesteal so i decided to post the same suggestion just came up late.
Also for long time ever since Yi rework i noticed that Yi actually like to kill turtles with that true damage and 
BORK
and somehow everyone even today think Rammus counters Yi but in fact Yi doesnt give a fk about thornmail anymore cause he does bonus true damage now. But i will leave them to think that :3
Meep Man1 votesIt should be more like:
NEW: Unique Active- Blood Thorns: For the next 5 seconds, Thornmail applies Grievous Wounds to enemies it damages for 1.5 seconds. (50~ second cool down)
This way, Thornmail doesn't just flat out reduce the target's life steal against you by 40% and instead provides a window of power to the user.