Remove AP and AD.

It's kind of pointless to separate AP and AD. Instead, why not separate basic attack damage from ability damage? Right now you have a situation where champions like Zed can do insane damage with their abilities, then follow it up with tremendous auto attack damage while they're on cooldown. You're not forcing anyone to specialize when they pick a champion. This would do away with "broken" champions like Yi who can deal ridiculous damage with Alpha Strike and finish off any stragglers with two autos. It would also force ADCs to decide if they'd rather be ADCs, or have the early poke their abilities provide, thus separating auto-attackers from casters. Right now, the word "mage" is rather loosely defined because so many assassins and even fighters behave like mages.

Have basic attacks fueled by one stat, and all ability damage fueled by another. Make players think about whether they want to do damage with their abilities or their autos. Make hybrid builds viable. Stop making tanks prioritize (as it is, you have enough armor-piercing items in the game that a tank with a ton of armor can be shredded in four seconds by the right pair of champions; why force them to build MR if the enemy Kat is fed?)

I think this is one of the best moves Riot could make right now, and it'd additionally make future champion design and item reworks a little less hectic to manage.