Do something like they did with Sunfire, make a buildable with a weaker version of the unique passive, or make it a "stackable" item. After deflecting X amount of damage, thornmail upgrades. (maybe with an additional unique passive or stat) Name of it doesnt matter. That's how you'd buff it for late game though.
Woodakoodashooda1 votesThornmail provides the largest amount of armor of any item in game and is relatively inexpensive. Its build path is "build cheap armor, build more cheap armor".
1 800 Top Lane1 votesYet an overall ineffective item lategame.
Woululuu1 votesMaby ineffective item in the lategame, but pretty strong in the mid game. If you buy that item you Want to make the adc totally useless in the mid game, but you give up power to do something against him in the late game. So strong mid game item but weak late game item. Don't know why you should buff the late game than.
I Blind Ferret I1 votesNot sure why people are hating on thornmail 