Could the problem with turrets be that they deal ONLY physical damage?
If you look at the boards you will see countless complaints about how weak turrets are right now.
Maybe the problem is that champions who stack armor can tank towers all day with nothing to worry about.
What if instead of dealing only physical damage they also dealt magic damage?
You obviously can't just add magic damage on top of the physical damage, so the physical damage would need to be reduced some and I would leave the exact balance ratios up to Riot to figure out through testing.
I also think it might be wise for all towers to deal a much smaller amount of their damage as true damage, making them always relevant, even against monster tanks. Possibly even make this figure scale on some really small percentage of the target's % total health.