We need a new stat for "attack damage"
Basically the idea is to split attack damage into 2 kinds of amplifiers.
One that increases your Auto-Attack damage. And one that increases your physical! (ability) damage. Kinda like AP already increases magic damage.
Why do I think, the game needs this?
League has a lot of different classes: Assassins, Bruisers, Tanks, Adcs etc. The main problem is the difference between Autoattackers and Casters.
And a huge deficit in Riot's balancing is, that they can't differentiate properly between those classes. Like: A good item on an assassin gets abused by adc's and just nerfed for this reason.
Now instead of giving that item 1 "ad" stat, give it an AA-amplifier and an physical damage-amplifier, for example: 50 AA-amp and 80 pd-amp. Since adc's need to AA more than cast skills, the item might be worse for them but still good for the assassin.
On the other hand, assassins are supposed to kill their enemys with skills and not AA them to death...
I know this would be a huge change and a lot of work on Riot's side but in the long run, it would make balance a lot better, I believe. This will allow more fine tuning and help design new items (for specific champions/roles)
Edit: What I mean exactly is that one "ad" item has BOTH stats at the same time, but with different numbers. Items designed for casters would have more physical damage-amp than AA-amp and vice versa tho. And some (allrounder) items might have the same value for both amps.