I know, I know, there's already multiple discussions of healers being useless thanks to the recent TOP victory without em but its really an underlying problem with the core design of the holy trinity and its just going to continue getting worse unless the dev team finally addresses it.
Here are the main issues with them that I personally see.
Tanks: Too much mitigation and self-healing. Warriors being a prime example of this where even the most casual players can see how OP they are in normal content. We need their mitigation and healing skills to be nerfed so that it doesnt fully take away a healer's job.
Healers: This has been said for so many years but lack of diverse support and damage with healers not only makes the job very boring but also redundant taking into account how well tanks can do their job. Two possible solutions for this is to either embrace the damage meta and allow healers to have more damage buttons or do what I would prefer and nerf tanks so hard that they cant survive without both healers and dps so that healers have a reason to use all of their bloated healing abilities.
DPS: Only issue I see with them is the oversimplification in their job design. Of course this doesnt apply to every dps job but a common argument thrown around the community is their lack of unique playstyles and the awful 2-minute burst meta. We can still keep them simple to play but there needs to be some bit of flexibility.
I'm not a hardcore player nor do I fully understand the game's meta but if a filthy casual like me can notice and agree upon a lot of the gripes with the current combat design then it's a real problem. The whole point of the holy trinity roles in MMOs is to give each player a task they themselves can only do in group content. If another role is outperforming the task of another role then the trinity is broken.
Either have a tanks, healers and dps, or just make every class role-less and do like what variant/criterion/eureka/bozja does and allow every class able to do everything.