"So what is this ridiculous Support role I'm talking about?" This is an optional role for limited jobs and even DoH/DoL to a certain extent become more involved in dungeons/alliance raids/trials etc.
When I mean optional I mean that duty finder related stuff will offer an extra slot exclusive to support roles. It will not be mandatory for them to join, so you can still queue normally without one. All they will provide is extra assistance if they end up joining.
A Support role that joins in this manner will only have their ability actions available to use. So weapon skills, spells and role actions are not usable during these instances. For example, Blue Mage will only have access to Glass Dance, Eruption, Off-Guard or anything else with an individual cooldown.
Support roles will be way more durable, similar to NPCs that join you on instances, but they won't hold agro and become targeted by actions or auto attacks. They can however, still get hit by AoE effects and be put under certain status effects and disables that is part of the mechanics.
Certain disables, like the Seduce effect from Ruby Princess, will give you a bonus action called "Hydaelyn's Light" that lets you automatically break free from these effects. This way it will not directly interfere with the other party members. They will also not be involved with loot rolls unless all the other party members do not want it.
"So they aren't too amazingly useful and kinda just there, so what's the point?" Both the party members and Support will gain extra rewards like bonus experience and gil as well as support specific drops like rare crafting ingredients if they're DoH/DoL. Limited jobs will still be able to do what they're specific niche that needs to be fulfilled. So Blue Mage will still be able to learn new actions from enemies as a Support role.
Both DoH and DoL gain new specific actions that are related to their class. Like Alchemist can apply health potions for allies or drop explosives on enemies, Botanist gains medicinal herbs that can cures status ailments or give slight buffs or debuffs, Culinarian can set up a small meal that gives additional stat boosts or they could just chop up certain types of enemies for extra ingredients.
Support roles will not have access to raids (like Bahamut/Alexander/Omega) and extreme/Minstrel's Ballad instances.
TL;DR Ramblings of a guy with a dream of seeing limited jobs more included in contents.