can we make that 15% damage returned 20% and can we make the damage returned work for on-hit effects?
In this way we can FINALLY have a good counter against those on-hit based champs that are driving us all crazy.
Since Thornmail converts armor into reflected damage it already has a means of dealing with on-hit champions.
Also: such a change of simply boosting the returned damage seems to have the aim of making Thornmail fullill the fantasy of " You got hit by all the abilities, got shot multiple times in the face, but since you bought this 1 item: you killed the guy attacking you".
There is no justification for such a reality to exist in LoL.
Thornmail just needs to be good at something that isn't completely oppressive.
Like maybe "curse" anyone who attacks you so their life steal, HP/5, and other such healing effects stop working for ~1.5 seconds after striking someone with Thornmail. That's basically the niche it fills as-is if you didn't rush it to stop top-lane split-push Tryndamere by just standing there. Once someone has life-steal it basically serves to completely negate the life-steal rather than actually do damage, and with how the damage reflection is calculated it's pretty much impossible to actually benefit from life-steal vs. Thornmail as-is.
Maybe also give it a Randuin's-esque active that fires thorns out, doing damage based on the amount of armor you have so you get a bit of that Sunfire Cape damage on a heavy tank. Maybe %HP damage because you're a tank and flat falls off after a while. Could also have the CD cool down faster the more you're focused with attacks to make you want to get Thornmail on a bulky front-liner and feel good about grabbing it rather than grabbing it on a back-line carry.
That way the actual reason to buy it is clearer and it feels better to use without being one of the most obnoxious items to play against.
I just don't see making it better/capable of landing an AFK as being a good change to the game.