Atmas suggestion (based on damage mitigated)
aim
- create tankyish offensive item with crit chance.
- dmg needs to scale with tankyness (not abused by marksmens etc)
attached picture
Text in it:
Edit: would build out of 
instead of just 
Cost around 2500/2750/2900? 200 hp (same as banner command) 40 armor 10% crit
Gains bonus crit chance % based on the amount of Damage Mitigated (differential between would be true damage and actual damage taken)
or instead of extra crit chance instead gain bonus crit damage up to 230% (+30%) does not stack with IE.
Can be expended any time you crit what ever you had stored up. then resets to 0.
or here’s how it would work every time you crit it expends the damage stored up. There is a cap as to how much can be stored. (+30% bonus crit damage)
Example (didn’t check the math): say for every 50 damage mitigated gain 5% bonus crit damage
this would work with this default 1/3 chance to crit at start of the game. if crits have a stacking system you can see when your next crit will be. (once in battle that is just like udyr’s phoenix or jax ult) so you can try and time it such that you auto right after you took damage if you know your about to.
new crit chance formula: http://boards.na.leagueoflegends.com/en/c/gameplay-balance/tbEWpbfb-new-crit-formula
