To me the main issues that make tanks no longer feel like as defined as a role is that power-creep was allowed to get out of hand, arguably because of a specific job which I think we can all easily identify, and that just set the tone for most of the other tanks to then not be allowed to fall too far behind. Rinse repeat that cycle over a decade and well, you see what happens. So ultimately we ended up with tanks being way too self-sufficient and too powerful defensively.
Couple that with player complaints over the years of jobs and/or content being too hard based on one-off bad experiences or players, leading to jobs and encounters being drastically simplified and you end up with jobs that basically play themselves.
Also, I feel there was a vast underestimation of the power of self-healing as a defensive tool where things like shields and % mitigation can reduce or at best nullify incoming damage but self-healing can potentially go beyond that and undo/recover damage that was done previously when the healing crosses that threshold of outpacing the rate of incoming damage. This makes it far more difficult to balance against other forms of defenses.
Healers then got screwed because with tanks being so powerful defensively, the whole point of the role basically got nullified. If your primary responsibility is to heal a specific player but that player can just keep their health up mostly by themselves, then what are you even there for?
First and foremost, to improve both tank and healer game-play, tanks need their defensive capabilities, mostly in the self-healing category, reduced.
Then for tanks, there needs to be more tank specific mechanics in fights asides from the occasional buster. More hard-hitting boss abilities targeting other players that the tank has to either pick-up or have to use their defensives they can target others with. More phases with adds or multiple bosses on the field at once. More forced tank swaps. More fights where boss positioning and/or location in the arena matters. You could have entire mechanics about the boss needing to be moved to changing parts of the arena such as something appearing in the arena that needs to be destroyed or it will explode and do a ton of damage but you can lead the boss over to and have it cast an AoE on you that if it hits the object it breaks it or does a bunch of damage to it to make breaking it way easier. Anyways, I could go on and on, but I think the point has been made.
As for healers, the part I mentioned for tanks about spreading the damage to other party members more that the tank has to help mitigate so that the party member will survive would also help the healer have more to do.