I humbly suggest buffing 
as a counter to AA champions, particularly because of the the rise of 
. The reflected damage should also apply a healing-reduction debuff. This change would counter the nullification of reflected damage by lifesteal, which is a core state on almost all auto-attackers.
Thoughts?
Clearly you don't understand how powerful Thornmail already is as a counter to AA champs. In its current state it's literally broken against them, it just so happens that you essentially need more tank items for it to truly be effective. FF is entirely a beast of its own, and attack speed is the most reducible stat in the game, with the strongest tank items being aimed at it. P.S. you would need to have 100% armor pen and 30%+ lifesteal to actually gain life AAing against thornmail and thats just to stay at your life without losing any. it takes close to 3+ lifesteal items to out life steal thornmail, or some insane in-kit champ sustain.
OP: this is how power creep happens.
You buff thornmail to deal with FF. Then you need to buff AA more because thornmail is now the meta and ADCs are now worthless so they need a buff. Then they become too powerful and you buff something else. Frozen heart or Omen let's say. Then those items become an issue.
I agree that items that only benefit one stat are a bit more niche in general, but making them OP without consideration is not the way to go.
As someone who loves Thornmail, this is way too much. The counter to thornmail is lifesteal, what you're saying would make thornmail as defensively op towards ALL auto attackers as the FF is offensively op. (Weird wording, just, you're ideas make out Thornmail to be as over powered as Feral Flare.) I do think it would be nice to help thornmial counter FF, though.
Maybe something like Feral Flare's on hit abilities WILL be reflected by thornmail.
EDIT: Also somewhere Riot talked about how they don't even like grievous wounds.
