Time Mage (Japanese original is basically "spacetime mage") would fit this well. It could be hard to fit classic spells like Haste and Slow into the game as it is, and they've already taken a lot of its stuff and given them over to Astrologian, but otherwise it's not a big leap to say that damage ticking at regular intervals that can be burst early for big damage would be a form of spacetime magic.
Geomancer could work, since it obviously can't work the way it did in single-player games basing spells off your current terrain. They could simply have spells themed off different biomes/elements. Mimic/Mime could also maybe be made to work here, but I'm having a harder time seeing how exactly.
Final Fantasy Tactics also has several jobs that haven't appeared in many or any other games that could be made to work with the DoT thematically. Mystic, Orator, and Arithmetician all seem like they could have some way to tie them into DoT spells somehow. Also, with less attachment to them from longtime series players other jobs like Summoner have, there wouldn't be as many people disappointed that Mystic, as an example, wasn't a DoT job before.
Another possibility I can think of might be Green Mage. Although this wasn't a job other than in one of the bad FFT sequels, they did reuse the idea of "green magic" for status affecting spells in other Ivalice games. There are other one-off jobs from those games that could work just because they have little identity since they're not in multiple games, like Illusionist or Trickster.
I don't like the idea of adding Necromancer when there are so many other actual FF jobs that aren't represented yet. Reaper just seems completely out of place already. I know that technically there was one in the FFV remake, but I still wouldn't put it ahead of real classics like Time Mage or Geomancer.
