New Caster

[Seraphor]

Geomancer would be my first choice too. Capitalising on ground based area of effect abilities, and Water/Wind/Earth elements.
But I think there's too much Geomancer in the game for it to be a job at this point. They never add a new job that's already been covered more than a merely mention in some dialogue, and Yoshi P has specifically said they're not doing any more 'classic' jobs.

However he has said he wants an eastern caster job. Onmyoji could be something similar but with a dark twist, although BLM is already meant to be our 'dark' mage job. Or there's Ascetic, which was briefly hinted at in the level 90 Physical Ranged Role Quest via Gosetsu.
So that's what my money would be on, Ascetic. No idea what it would look like as a job itself, but I'm assuming a DPS equivalent of WHM, hopefully still using Water/Wind/Earth elements.

[Mithron]

and Yoshi P has specifically said they're not doing any more 'classic' jobs.

Source? I don't remember him ever saying that. I remember him saying he's interesting in doing other jobs in addition to classic, but never "no more classic."

I would 100% be down for Mystic Knight! Depending on what aesthetic they were to go with it it could either be Casting (therefore butting heads with RDM, so unlikely), Maiming (for a conventional knightly look, would have to skip next expansion because we just got a Maiming Job) or Scouting (which... could work, actually, it'd complement NIN as another melee+"magic" Job). We're due both a Casting and a Scouting Job, so it could go either way!

Scouting could work if they do the FF5 style of Arabian curved sword style look, similar to Minwu from FF2, or BLUs from FF11. But DEX is a weird stat to fit for melee spellblades lol