Riot wants thornmail to only be good with lots of armor? Fine, here's a thornmail rework idea
Cost is the same as live with the armor being slightly reduced. Its passive is changed to this:
Enemy auto attacks to this target take damage equal to 10%(+10% every 60-80 bonus armor, max 40% more) of the damage they would have dealt to their target before resistances as magic damage.
You can't just buy this item and be good like old Thornmail and it isn't useless without 500+ armor like current thornmail. This version of thornmail would be a piss poor rush(unless you're Rammus) and isn't truly good until 120-160 bonus armor making it a great 3rd or 4th item. The cap is there so that we don't have a malphite with 600 armor and 2k health have adcs 2 shot themselves on him(it?).