Chemist is what usually comes up the most, but because it's stupid, it'll likely be mixed in with another job.

Oracle was a job teased by Yoshi during two interviews as something that was intended to be a third job for Stormblood as an "Eastern-themed spellcaster", meaning that there could potentially be assets for it still in place that would make it easier for devs to work off of.

Geomancer keeps getting asked for by players way too hyped from its version in the Astrologian 60-70 job questline to realize that it's literally just the Conjurer base class with a different coat of paint and seem to frantically scrounge for any possible scrap of an argument to keep convincing themselves that the job will come out. While I wouldn't mind seeing Geomancer in FFXIV, I'd much rather see it get the Astrologian treatment, where instead of receiving the Hingan version seen in Stormblood, players would get a different version involving a story showing how the Geomancers seen before knew absolutely nothing.

Whenever someone talks about wanting Beastmaster, all thoughts sadly seem to point to that "lion tamer" circus reject version from XI. If it were to come out, I'd much rather see it as something that calls various beasts in to deal indirect damage. Like Ace Ventura in that second movie, except maybe using something like a warhorn instead of his notable butt-yelling in that film.

Puppetmaster is another job I'd much rather see re-imagined. Lord of Verminion sucks, so why on earth would you want it as a player job? In fact, it's because of how much it sounds like basically playing Pokemon that I'd expect it to be the next Limited Job to be released and being relegated to just a more advanced Gold Saucer minigame. If devs do decide to release Puppetmaster, my only hope is that they look at the XI version and think "that's stupid, let's make something different" since that's the only feasible way that Puppetmaster would not be a Limited Job.

To answer the question, the simple answer would be something demanded for fan service, and something different (if not wholly new to the series). If it's just one job, likely a mix of both.