Geomancer was teased in the Astrologian quests in Stormblood.

In those quests the geomancer NPC you work with uses basically the white mage toolkit but with different names, and is slated as a healer. This could always change in the future, but based off what's in the game now, it would seem that's what they were setting up.

Adding to that is more speculative, but they removed the elemental spells from white mage this expansion (no more stone and wind) which opens up that spot for a geomancer healer to take them over.