Here is how I would make tanks actually be decent late game:
Right now tanks full off extremely hard in the late game due to the amount of %penetration adcs get. However, they have an extremely strong midgame due to base damages and cheap items. What I will do is make tank items more expensive, but provide better stats. Another idea I thought of is to actually remove %penetration from the game. I know you would think that it would make them unkillable, but I suggest we reduce the base damage of tanks. This will encourage staying alive more to cc the enemies and actually make them feel like tanks. Right now tanks feel like season 3 bruisers.