What about creating an ACTUAL Dark Knight?

[MaraD_]

Yes, does that make Dragoons and Ninjas tanks, yet ?

In the context of FF4, yes.
Blink was broken as heck, and Jump allowed Kain to survive Big Bang in playthroughs where you were severely under leveled.
DRGs are the most tanky jobs in all FFs with their ability to have higher than average HP, good armor, and a 100% evasion skill that also does dmg.

Put either kain or edge in the back row and they then take -50% melee dmg, while also doing dmg in the back row. (edge does more dps, due to jump taking up a turn, lowering the actual DPS. but DPS doesnt matter in a 1 on 1 scenario w/o a DPS check. As the turns in which a boss wastes attackign nothing, equate to the turn having never happened.)

Put cecil in the back row, and he's just a bad healer. (i tried this for my low level play through, he could survive big bang on occasion, but was too useless back there.)
Kain and Edge were beasts as tanks compared to cecil, when maximizing potential.
Cecil is just more straight forward, and "easier" to just take hits, since there's no set up for him.

Provoke and Cover are skills designed for you to take hits instead of you party member, that's the concept of tanking outside of MMO, and it existed since FF III.

Provoke yes, though useless outside of FFX. Cover, not so much, since its too rare of an occurrence, which cant be properly controlled. Its better suited for offense.
put on a slow character with massive attack power, given counter attack, then put in a MNK who can easily get to low HP. (like FF6)
It COULD act as an actual means to protect allies from damage in the future games, like it does in XIV.
But you'd protect the squishies far more by sticking them in the back row in FF4. Putting non squishies in the front row. Thats the most effective tactic for protecting them from dmg.
tanking fundamentally from game design, is the job of being defensive. healers and tank are technically the same role, from a game design perspective.
but like how melee and casters are sub roles of DPS, tanks and healers are sub roles of defense.
so simply lowering dmg others take, is the equivalent of tanking (as its mostly mitigation, and someone being dedicated towards mitigation. while healers are less about mitigation, but HP going back up.)
So While Cover and Provoek are "tanking methods" its not from directly mitigating dmg, but indirectly mitigating dmg.
The spell of Protect counts as mitigation and "tanking" and so does having to force someone to the front row, where row slots are a limited resource.

So in the end, yes NIN and DRGs tank.
(also depends on the game too, as both are more than qualified to be designed as such. Just like most jobs)