So most high dmg champions this meta get most if dmg from spells, which are ranked up early and deal dmg from the spells base dmg and late they scale with items. But adc abilities are weak to compensate for their supposed basic attack damage which is lacking in the early and mid. And although they have base dmg his base dmg cannot be upgraded like the abilities of a mid lane mage, but what if it could? Imagine they same max lvl for all champions but adcs coukd use 5 points on auto attack levels instead of a useless ability early game. This way their basic attack dmg gets an early boost but is limited from being levelled ip out of control late game and if any other class wanted to lvl up basic attacks they would have to sacrifice ability points.
I have not yet thought of a good solution for bruisers not benefitting from this but i would suggest reducing their damage gained per level a lot in the same way, for example: fervor grants two stacks for melee attacks or phage grants less move speed for ranged attacks. Suppose a level granted 5 ad for ranged champs and 2.5 for melee champions so they would be pushed to level up abilities like usual.