NINJA and ROGUE confirmed

[Iriadysa]

By your explanation, MNK/NIN should have been a tank.
Enmity generating ability : Boost (Check)
Mitigation skills : B+ Evasion, A- Guard, Utsusemi from /NIN (Check)

I wonder why we haven't seen that more

All jobs were capable of a limited ability to tank once they were able to utilise Utsusemi: Ni in addition to Ichi, however, they'd not last too long because they lacked the inherent defensive skills of Evasion/Parry at a satisfactory level in addition to having gear (unless they chose to have an evasion set) that supported that role. Ninja gear in general contained Evasion/Parry/Enmity bonuses and the later gear also added +1 shadow on Ichi and Ni casts yielding 4 and 5 shadows respectively as opposed to the 3 and 4 originally (3 for Ni if subbed by any other job).

Additionally, you're conveniently forgetting the original point of those defensive abilities have to be backed up by solid Enmity generation where Monk, Corsair, and Puppermaster fall short. I'll give you Thief because people have successfully used it to tank content such as mini-bosses in Sky with full evasion sets but it generally lacks the gear to support such a role without sacrificing it's primary DD role.

Neither of you seem to have any clue of what you are talking about, so let me give you a hand here. Note that I'm talking about lv75 content before Abyssea. I left XI before it.

MNK was a tank. Did none of you ever do low man Salvage? I can't remember taking a NIN or a PLD for anything at all, it was good MNK tank all the way. You could easily make MNK tank other relevant content if the MNK knew how. Low man tier 1 jailers in Sea, Limbus, anything in sky, etc. Not only MNK was a tank, but SAM/NIN count tank just as well, if not better.

Once you had utsusemi, the only requirement for tank was enmity generation. Proof of this is that MNK tanking was done with the counter stance ability up, which droped your defense to pretty much nothing and yet it was still a fair trade because you could counter while casting utsusemi. Enmity generation for MNK and SAM was done through sheer damage, so you needed to be well geared for damage there. If for some reason enmity was a problem, a good THF would remedy this easily with Trick Attack.

On another note, RDM was an incredibly good tank (specially if you had a way to secure enmity on it), much better than a NIN.

Utsusemi and enmity became synonymous with tanking, not NIN.

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On topic:

Yay, NIN is coming relatively soon, could be fun.

THF, as designed in FFXI, is a really interesting concept. It's an enmity juggler without being a tank, which opens up a lot of alternatives when approaching content. However, due to how easy and simplified enmity is in FFXIV and the availability of skills like Provoke to rapidly switch enmity, XI's THF concept seems to be redundant for FFXIV and as such it would just end up being a reskinned NIN.

Without special mechanics to the job there's just no place for it in this game.

If it ever actually happens, I'll expect it to be a support DD/debuffer of some kind and if so, we won't see it until proper support jobs are introduced. Assuming Yoshida and Co. are still going to do that.