I think it's time Riot adds a third primary stat. (AP, AD, PD?)
I don't think AP and AD are cutting it anymore for champion balancing, I mean they've been around since the beginning with only 16 or so champs. We're at a hundred now and forcing champs and items around this unchanged system is starting hurt the game. Specifically with fighters becoming tanky AND brutal in damage, because if we change the items for the ADC the fighter wants them too.
Why not add a new stat, split up AD as Force Damage and Precise Damage. (Force Damage stays red, Precise Damage is now yellow)
All the crafty, swift and cunning archers, assassins and ninjas now get ratios from Precise Damage or PD instead of AP/AD. It always kind of bugged me that characters like Nidalee and Akali both require AP, when nothing about their attacks really scream magic to me. Yes magic is involved but seriously it's not a beam of light they're shooting, they're stabbing and biting and chucking spears and knives.
PD would still counts as physical damage, hence still affected by armor. Hell the glyphs are already set up for it, with red being the FD, yellow PD and blue AP. Any trinity force could actually have 3 different stats, with the new PD on zeal.
TL:DR, create a new source of damage, adjust ratios of champs that are archers/ninjas to use this new dexterity ratio, solves Fighters wanting the same items as Marksman somewhat. Still a bit of work but healthier in the long run.