While it's true that yes, Kain obtained the Holy Dragoon Job in FFVI: The After Years, it was more to fix a discrepancy, in that Kain Highwind had surprisingly low defensive stats compared to Dragoons later in the series, such as Cid Highwind, Freya, and to some extent, Kamahri Ronso. Additionally, while its true that in the MMOs, Dragoons generally are damage dealers, when they have access to the Dragon command, they tend to lean towards the concept of tanking in single player games. Reis, the first to have the command (by being a dragon, don't ask), could outside of using various elemental breath weapons, heal her allies and remove debuffs from them at the cost of some HP, charm enemies, grant increased stats, and grant haste. Freya extended upon this, with being able to cast a multi-target regen, restore her and her allies' MP while damaging the enemy's, inflict her allies with berserk (which also vastly increases their physical damage), and, by luck, she could potentially fully restore her entire party's HP and MP. While Tactics Advance and its sequel made them raw damage, Bangaa Dragoons had some of the highest defensive stats in the game. Finally, in Dimensions, we see a return to form, with powerful defensive stats, and their Dragoon command giving them access to party-wide healing, a stun, and a sacrificial move that revives their allies with full health.

What this past boils down to, is that Dragoons have, for a very long time now, possessed a strong supportive role backed by incredible physical and magical durability and strong, but not overtly so, damage potential. So, while yes, we see them as damage dealers here, Dragoons have the pedigree to be a tank, and, honestly? I think if we do end up getting compressed skills and "skill trees" in 4.0, we may see a tanking style for Dragoons to give us a fourth tank, probably and HP-based one at that to complement Warrior, much like Dark Knight complements Paladin. This would also open up Paladin and Dark Knight to being the durable force of destruction they're known to be, and give Warrior the ability to be fully offensive. I wouldn't be surprised if FFXIV takes a skill-tree route and gives us more Jobs based on the 13 Jobs we have now, if they took each Job's skill set and gave them new abilities for a different role and a different name to go with it, we'd have 26 Jobs, and each of the 13 Jobs we have now could easily be modified into their own base class. But that's a discussion for another time, bottom line is: Yes, I could see Holy Dragoon becoming a thing.