They should just be given a passive called 'Lethal Damage'
Turret shots pierce through any and all shields (through Locket, etc.), apply mortal wounds, and deal scaling % max health true damage.
Oh no Cho'Gath is trying to tower dive, 10% max HP true damage, 20% max HP true damage, 35% max HP true damage ====> 40000000000000 HP Cho'Gath has been slain after only a couple more shots.
Towers should be impactful, they should hurt like hell to try and tower dive, it's a tower, why exactly would it be so weak. Make the game have some strategy to it again, and make it so that improper rotations punish you very hard.
In addition, base-tower armor should be reduced, however for every allied player nearby, that armor should increase. With 1 player, it would be even to where it is now, with two it would be slightly stronger. A tower with 5 people guarding it would have extremely high resistances.
Why would this work? Split pushing would become far more potent and powerful. A team that doesn't react to an enemy splitpusher would find their towers toppling, whereas a team who opted out of having a splitpusher would struggle to as easily take their towers (as typically they have a much, much greater teamfight presence as a result; i.e. Tryndamere vs Galio).
Of course this isn't a perfect solution, but it would work far better than the current system in which you can pretty much just ignore the towers. I know as Riven I intentionally trade the enemy under their tower, and just shield the tower's retort. The tower deals no damage to me b/c of my shield, and I'm getting the enemy lower and lower until I hit 6 or get my ult up, and just faceroll them.
As Ornn I just slapped the enemy Olaf around all match, taking tower shots whenever he was stupid enough to try and go for a minion, as my shield would absorb all of it's damage and I'd chunk him out. It was honestly kind of ridiculous.
[sg-soraka]