I think this one point here is the crux of everything. Tanks just need more to do. We now all have tools to mitigate damage on other people, but as far as I am aware, they are mainly used on the other tank. They have used mechanics where the tank has to be in front to take the most damage before (Thordan EX comes to mind) but it really is rare and I don't think it has come again since then (from memory).
One thing I will say though with the example of the AoE, only healing enough to survive and then the boss throwing out another, the problem with things like this is it could be seen as a punishment mechanic. As a healer. you knew the party would survive, they do, but the boss decides to throw out another anyway to kill everyone, whereas if you had healed more before the first, you wouldn't have the second AoE. It could be seen as a punishment as you have to spend more resources before/after to ensure you stay above the HP threshold rather than knowing you would survive and allowing your regens to do their job. This is where you have to be careful with mechanics as you do not want to feel like you are being punished by taking more damage for no reason.
