I feel like a "support" role could be added if they kept it tied into raid like content like Coil. However pulling this off wouldn't exactly be easy since there would have to be a way to make these roles applicable in the traditional role system in place for dungeons.

Doing something like well add a 5th slot wouldn't help since you can't magically change all past dungeons to have 5 members nor can you alter it where that new role would work for those dungeons.

To make the new role work you would have to think of it in a present tense manner and ignore the past while producing a manner to make them perhaps fill a DD role in older content.

With the mention of chakra up above a potential way to do this would be the following manner.

So we have all these DD roles in the game atm, which means that a potential idea could be to allow these DD roles to change their skillset to "Support" role which would change the dynamic of how they play compltely and tie mainly into doing raid content.

Let's use Mnk and chakra/chi like concept, perhaps their alternative role could be "Chi master" and their main gimmick would be the ability to remove/imbue special attributes onto players aka maybe adding that whole elemental barrier into the game, so you are fighting a boss in a raid that uses a poweful fire attack. The support role of "Chi master" would have to pay attention to the cues and in order to survive the attack they would have to use their support to aid the raid otherwise and instant to near instant wipe aka hard to recover from. But the thing to keep in mind is when you add in a new role you have to consider how it will play out in content, you can't add a role and magically it works to make the game harder the game has to be designed with that role in mind and that's what makes it so hard to just add a new role is the fact that mechanics have to be put in place to make it relevant.

Currently the game works as it does because there are cues to keep in mind for DD to play an active role in keeping themselves alive and limiting the amount of healing that needs to be done. The healer keeps the party alive and the tank holds the mob to absorb all the damage. It works out nicely because each role does play a pivotal role in keeping the battle flow stable. If a DD gets hit by an attack they could of dodged it throws off the flow of dmg output, the healer has to raise in order to regain that dmg output and in turn they lose the MP that could of be used to keep others alive. then of course the tank that fall back on the other tanks to make sure stability isn't lost when a tank goes down, reason you have two but at the same time the 2nd is needed to take adds as needed but overall the game is designed with the thought that all these are needed.

Stress is relieved from both the tank and healer by having two in the party so as a healer you aren't auto screwed if they die, you still have the other that can keep things stable and get the other one back up if something bad happens but at the same time it is hard to keep it stable when it happens but it's not impossible.

Adding another role just makes it a bit more complicated to create content that is challenging and keeps it all in mind while it's possible there are just more variables to consider and keep in mind from a balance point of view.