I agree with warrior and kinda gunbreaker because they were the only sorta tanky ones in the games and at least there's semi hints of it in the games even if they weren't always exactly a tank IE beserker /fighter.They sorta threw a few together to make warrior. Squall had story hints at it with his dialog. At least, I guess they kinda work and I can see where they may want to base it from. A lot of the base classes in XIV were tanks in other games anyways.
Dark knights were pretty much like dragoons. They had weaker defense stats than dragoons and less health. No tank type utility either. They have to had gear since they are considered a Melee class in general and not evasive so they would wear heavier gear besides asian themed classes like ninja/blackbelt/monk etc since they were more speed focused and less heavy attacks. Also Cecil had low magical resistance as a dark. For stat weights, Cecil as a DRK at level 20, he had 520 HP, 21 STR, 15 speed, 16 stamina. At 30, 920 HP, 29 STR, 20 Speed 21 stamina.
kaine at 20 had 523 health, 24 STR, 18 speed, 17 Stamina. At 30 he had 963 health, 33 STR, 25 speed 25 stamina.
In FF3 darks had over all the worst defenses of the melees but had over all comparable stats to DRG for damage and they both had no tank utility. Also, DRK's often have dealt the most damage in general in a lot of situations . Knight and vikings had utility and defensives Vikings had a provoke.They also had White magic in famicom version and souleater in the the 3ds verison cleaving AoE damage that took health. Drks were mostly used for the massive AoE and katana's being strong against enemies that divide.
In FF5 DRK's weren't a thing but they also had similar stats to DRG's and wore Heavy armor like DRK's in other games and DRG. These classes wearing heavy armor instead of medium is just mostly for aesthetic reasons and balancing and for it to not be a speed type dps for dps diversity.
They are really cutting into a lot of the melee classes with very few left. The next tank I could see is viking. Unless they merge Viking and something else like an elemental Hammer wielder melee DPS. Mystic knight could be a tank too but it was more melee dps like dragoon and dark due to its lower defensive stats and health like dragoon but it has more agility and magic stats. Sadly classes could also somehow be considered tanks too. Since it had a defensive buff and was rune fencer which was a weird one. Ffta2 used spellblade and steiner was like a mystic knight but he was also tankish.
I'd LOVE it if mystic knight was some sorta dual wield class though and merged with another melee dps class and used Dragoon gear though and given a new name. Mystic Blade or something. Or maybe more generic since they may not use a blade.
Kind've a shame since I like melee dps but not a fan of Asian themed classes so it's either dragoon or tanking. I personally don't like tanking all that much however.
Really, they will put pretty much any class into that position if it's:
1) Hyped up soon enough that they can start developing for it.
2) It's a role they currently need.
So, if people were hyping up a gunbreaker class during sb/shb xpac instead of in HW and into SB, we probably could've seen it as a mDPS in 6.0. Same with them wanting a tank in 3.0 and DRK was the most hyped class at the time since ninja came out besides SAM and RDM and SAM felt more like a DPS to them as well as wanted to wait for 4.0 since it fit better. The only potential melee dps classes I've seen hyped up a lot are ones that are mostly able to fit in the limited job category to them. there's a high chance beast master and puppet master will be limited jobs (I don't really care for the jobs anyways) since he's mentioned that they'd fit better in that aspect.
To answer the topic, Probably not. I really can't say since it's play style has to be fun for me and it depends on where they want to take tanks in 5.0 with all the changes. Can't main a class based on aesthetics alone or just from a few animations we saw.