I feel like different type of attacks can be more effective against specific type of mitigation would solve multiple problems:
When I say multiple attacks means 2 things: Raid-wide attacks, Tank busters.

What I mean by different type of attacks:
1- Attack 1 that countered 100% by Shields and 50% effective on defensives and blocks.
2- Attack 2 that countered 100% by Defensives and 50% effective on shields and blocks.
3- Attack 3 that countered 100% by Blocks and 50% effective on defensives and blocks.

How could this solve tank/healer issue:
1- Tanks will not as op as we see now.. Warrior can't mitigate Attack 1 as how DRK can mitigate it 100%.. team collaboration will be much more important.
2- Healers can finally use their single target mitigation and can be meaningful because if we have WAR and DRK and boss perform Attack 3 it will require healers to use their "block" mitigation to have it full 100% effectiveness.
3- Make tanks more specialized and require team synergy to mitigate attacks.. when I say specialized it is more like DRK have most of their tank utility as shield but also have block but less than PLD for example.

but this require tanks to be more specialized and have to get full rework on mitigation part.