Would it help the ADC class to get a +10/15 AD on Infinity Edge for a minor cost increase?
Autoattacking ADCs rely on IE/Essence Reaver as the main AD item for a good chunk of the game (5.200 worth of gold for the two Zeal items) while lacking a good damage Keystone.
Adding some more AD to the item completion while raising not the cost too much would give ADCs more AD to multiply while building Zeal items.
Something like this or give Fervor of Battle some love, making it a decent sustained damage source in all phases of the game. I'd personally add %max health on-hit damage to the stacks, so that tanks can't get hit for 10 seconds worth of autoattacks in the mid-game while suffering less than half their health in damage.