What's next for new jobs after Endwalker.

[Seraphor]

[on Onmyoji]

Onmyoji are already mentioned in the game in some offhand comments. (I forget where exactly) but in the same way that Geomancy is likened to being 'similar to Conjury', Onmyoji are similar to Thaumaturges. It's the same sort of relationship, with Geomancers being in some sense opposed to the Onmyoji and vice versa.
Ultimately however, the job is basically Mystic or Oracle with a different name, so there's still a sense there of it being a classic FF job in some ways.

Then there's also the Ascetics, yet another eastern mage class that Gosetsu becomes in the level 85-90 Physical Ranged job quests.

There's no shortage of eastern mages to draw from, one of them has to be our next caster.