A reasonable Thornmail Buff.
Serrated Spikes Active: For the next 10 Seconds anyone who takes damage from Thorn-mail has Grievous Wounds. (30 second Cooldown).
We have Grievous Wounds for AD and AP how about a tank.
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As it has been mentioned many many times on the boards, and as it has also been said many many times on those threads: #Grievous Wounds was meant to combat lifesteal as was Thornmail.
You can't stack the two otherwise, tanks will be able to just stand there with 400 armor against full AD teams and get a penta.
isnt meant to be a damage item
when will people understand thornmail isnt meant to reduce healing? it was specifically designed to counter lifesteal.
which it does fairly well, just not many champions make use of lifesteal anymore.
which has brought it to a niche item where the enemy has a few ad reliant champions.
Nah, let's not do that
I'd also go for true damage
i would propose a 1,5 second grievous wounds debuff
that would stop lifestealing
It is too easy to buy one of these 
and render the return damage it ineffective, or simply pick warlords bloodlust, which when combined with the 30 base mr would already outheal thornmail
Lifesteal is very accessible as is mr these days
Other options include to make it also scale with bonus health so you can make it scale up faster and harder
But at the very least it should get 200 health, thats very easy to accomplish by adding 
in its build
Basically what i would like to see is this
+ + + 850 gold =
2900 gold
100 armor
200 health
Unique: Reflects 15% of the physical damage taken (+ 25% bonus armor)(+2,5% Bonus health) from basic attacks and on-hit effects back to its source as magic damage (Calculated before any resistances or reductions are taken into account).
well you get the idea, this way it would be competing with the other 3 big armor items, except those offer more health and this one offers more armor.
I propose it specifically target lifesteal instead of all healing, otherwise this would be a huge nerf to juggernauts who interact with tanks on the frontline.
etc.
I think if the item cost was raised and it offer a bit more armor AND one ruby crystal it wouldn't be that bad of an item for a tank.
Right now there are just better tank options and no one should be building thornmail beside a couple champions.
should all be built over if you are a tank over .
I am surprised it is even on the recommendation page for any champion. It is one of the worst items in the game just because there are way better tank options.
Edit: Oh I will even add 
AND are BOTH better than thornmail as well since they cover two resistances and both have a passive that is relevant.