Critical strike should be changed for Armor pen and Max HP dmg for Marksmen!
Honestly, I always hear the lame excuse "but...TANKS!" when I point out that the average ADC 3 hits every non tank character in the game, because of how broken crit is.
Just make crit melee only and give Marksmen a wider selection of Items that make them ignore armor or deal more dmg when the enemy has alot of HP! Bam, they can still kill tanks without having a 600 dmg ranged nuke as an Autoattack against everyone else.