thormail idea (runicblessing inspired)

TL,DR: +500gold enchantment to thornmail. Gives true damage shield when exiting base like the runicblessing mastery that no body uses. can only be purchased once the game exceeds the 40 minute mark.

so it occured to me that thornmail is only 2100 gold. and i figured it must fall off late. correct me if im wrong. im no wiz but just seems to be dollar per dollar in flat out trades when it comes to item slots id rather have a trinity force or randuins, banshee's, spirit visage in place of thornmail. assuming the enemy has equal amount of mixed damages. unless of course your name is Taric.

ANYYYYYYWAYS: so i challenged myself to come up with a possible buff that would be half decent. I was about to make a thread and asking the board to brainstorm some ideas. here's what i came up with after briefly dwelling on it. It would be an enhancement (much like enchantements to the jungle items) ... now i just had another idea. what if thornmail could have like 3 enhancements to build out of. okay getting sidetracked. so the enhancement could only be unlocked after the 40 minute mark for those long drawn out games (not sure if this will help extend length of games or shorten them)... at 40 minutes the enhancement you could buy for just 500 gold would be that you'd get a shield to true damage when exiting the base. it wouldn't come back up if destroyed unlike malphite passive (even if it could replenish itself it wouldn't be for a long time more like a zac passive). just saying as a tank getting 1 shot by chogath can be pretty annoying.

it'd be kind of like the armor equivalent to banshe's except only respawn when you exit base. the shield would only have about 200 to it. and if enemy team has no true damage you would just die without ever having benefitted from it.