What if ADCs got scaling damage?
Instead of Crit?
Every 1 second you're in combat against champions you gain a stack of Marksmanship, stacking up to 10. After 3 seconds of not being in combat against champions your stacks rapidly begin to fall off.
1 stack of Marksmanship increases your damage by X based off of your Marksmanship stat.
AS could increase the speed at which you gain stacks.
IE could be reworked into an item that lets you stack extra marksmanship stacks, or gives you a base amount that either never falls off or gets instantly applied upon entering combat.
Suddenly ADCs no longer melt their counters; mages and assassins but still do their job of killing tanks and objectives.