What about creating an ACTUAL Dark Knight?

[Reynhart]

that is a rule you created, and isnt actually a requirement.

It's as much a requirement as having a tank in the party, which may not be necessary if you can indeed burn the content faster than you die. Fun fact, I did FF I, III, VI and VII as an "Attack only" challenge, meaning you only can do normal attack over and over in combat (Limit Break are allowed for VII since they "remove" your attack command). And yes, in those games, it works, so you can skip having a "tank". It's still the primary role of a tank.

im a bit confused here, maybe im miss-remembering FF4, but thought cover only worked when they were at critical hp, and not at full HP.

I've double checked and even in Final Fantasy "II", Cecil had a Cover command. The critical part is a passive trait. In FF III, the Knight has only the passive Cover for critical HP.

but on a side note, im not suggesting PLD cecil isnt a tank, so much as the way SE designed the game, all melee were capable tanks by the standards SE set up, but not necessarily by the standards MMOs eventually set up.

Except that again, other frontline characters weren't able to do any "tanking", and DRK Cecil had comparatively low stats, making this job clearly not a "high def, high HP" job...on top of slowly killing himself. But again, I don't say that this concept couldn't be used for a tank, just that, before XIV, no DRK had actually real tank attributes/mechanics compared to other jobs from the same game.

Most MMOs copied those features, so even in games like FFXI, where they wanted all melee to take turns tanking

They didn't. FFXI was very restricting in the tank department when it launched. Only a PLD/WAR could tank properly since PLD was the only one that could survive at high level, and WAR was mandatory for Provoke. in Zilart, they tried to make SAM the next tank but they didn't realized how broken they made Utsusemi, so NIN took the role. Only years later, you ended with almost all melee that could tank, because 1) Damage enmity was absurd and 2) healers were overpowered. Fun fact, when they crippled enmity from damage, it almost killed every tank except PLD/WAR. (Yet again, it might have changed since, it's been years since I didn't play XI)