Give turrets armor pen equal to base armor.
And make them actually deal damage if you dont build armor. Champions shouldnt be getting away with building just HP And turret diving safely for 4+ shots mid game.
I'd say have turrets deal 25-40% max HP physical damage, and have some amount of %bonus armor pen to prevent immortal Malphites/Rammi. No armor? Get rekt. Have armor? Good, you're supposed to if you're turret diving or tanking a siege.
http://preseason.na.leagueoflegends.com/assets/img/gameflow_turrets_screen.jpg