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I was thinking about the problem riot is having with ad casters (mainly the problems with not having spells scale as well as they could because you have to worry about the power that the items also adds to the auto attacks so the spells need a high base damage and no scaling and everyone just builds tanky).
My tentative suggestion is to give everyone in the game an ad ratio on their auto attacks. This would allow riot to tune one more thing to bring balance in line. You could leave ad carries and most other champions around a 1.0 ad ratio. But you could change physical casters like Renekton to closer to a .5 ratio. This would let you shift more power into their spells which would allow them to feel good about buying offensive items.