Geomancer would make sense. Fits well with the conjurer and gridanian lore.

Time Mage wouldn't be a bad idea, however since Yoshi-P said they're looking at adding a caster class next (chances are it would be arcanist), I'd be tempted to associate it with arcanist instead.

Unless they intend to associate Summoner with conjurer, too. After all, in past FF games such as FFIX and X, Eiko, Garnet and Yuna all are summoner/white mage hybrids.