Built in Lethality
What if Riot nerfed the amount of Lethality gained by items, and added some mechanic that added a flat Lethality by level, rank, or some scaling mechanic, that put champions who are meant to utilize Lethality up to where they are meant to be.
So, as an extremely general example, ... items like
could offer 10 Lethality, instead of 15
Champions like Kha'Zix could gain a bonus 50% Lethality from items.
I don't honestly know if this would "balance" things, but I think this would make it so that champions who are not really meant to abuse Lethality could still use it, but to a lesser degree of effectiveness. Meanwhile, champions who need the stat can still gain the full intended effect, or various degrees of effectiveness based on the individual needs of the champion.