Crit idea
We have Last Whisper and Black Cleaver for armor stacking tanks and Void Staff for MR stacking tanks and they're all rather effective.
For HP stacking tanks though, there aren't a lot of options and they aren't really effective (BOTRK, Liandry's Torment and the Giant Slayer passive).
What if Crit damage scaled with the amount of bonus HP?
The Crit Chance could be either identical to what we have, or disappear completely. If it's the latter, then stacking crit items would increase the rate at which damage scales depending on the enemy's bonus HP.
This way, it still allows ADCs to DPS every target without being able to burst squishies like a mage or assassin should.
TL;DR: make Crit a Giant Slayer.
?
and 
line into utility like giant slayer working vs anything with more hp
would then buff both damage and pen