How would you tackle job identity?

[GoatOfWar]

Start introducing weaknesses.
To give you some random examples..
Dark knight can't heal at all and requires healing from others.
Warrior requires external mits to survive some tankbusters.
Paladin does less damage.
Gunbreaker has no partywides.

If we want job identity to matter, they can't all do everything.
In order for there to be strengths and advantages, there also have to be weaknesses.
You can't only have advantages, because jobs then just become overpowered. Like Warrior. Yeah, it has advantages.
But it has no weaknesses to make you consider other tanks, so everyone just defaults into playing Warrior now.

[Aniya_Estlihn]

Dark knight can't heal at all and requires healing from others.

It already has that issue with the recent expansion's homogenization, quit playing around.

[Shurrikhan]

If we want job identity to matter, they can't all do everything.

Yes, but there's a heck of a difference between having only a small number of the total pool of special features... and lacking basic capacities.

...And there's a large difference between lacking a redundancy and an actual weakness.

Take Dark Knight, for instance. Putting aside denying a vampiric tank any self-healing, unless that lack of healing comes at cost to actual sustain (its means of staying alive longer, without reliance on kiting or externals), then it just means that it's all barrier, no healing... which is actually... better 90% of the time anyways. Or if it does come at cost to sustain, then it'd need some capacity to be advantaged in compensation, and we've already seen what happens when that's damage. (At best, the tank is exceptionally fragile at start of tier only to become thoroughly meta once geared.)

Like, in terms of overall vibe, I agree, but... there's a majority chance of a Monkey's Paw outcome when following some of the examples you've given so far (especially DRK and PLD, and GNB for any content tuned around tank raidmiti).

Definitely agreed regarding Warrior, though. If we don't want 1 tank to be consistently dominant, we can neither afford to leave it the best at everything nor "buff" (read: simplify, inflate, and break role balance to prop up) the others up to its level.

[Taranok]

Warrior requires external mits to survive some tankbusters.

They did this in ARR. The class was so garbage that the very first patch ARR got specifically turned WAR into PLD. This is the first piece of homogenization the game actually got, and kicked off the largest source of homogenization the game has seen. That is, turning all tanks into paladin. The second piece happened in HW, when all healers were turned into white mage. This decision has had knock on effects that last all the way to this day.

[GoatOfWar]

They did this in ARR. The class was so garbage that the very first patch ARR got specifically turned WAR into PLD. This is the first piece of homogenization the game actually got, and kicked off the largest source of homogenization the game has seen. That is, turning all tanks into paladin. The second piece happened in HW, when all healers were turned into white mage. This decision has had knock on effects that last all the way to this day.

Then there will never be a job identity ever again.
If you don't want to have weaknesses then you can't have advantages.
Otherwise your job just becomes the best, like it is now.