Geomancer cannot be a new job, be it tank or not. This is established in the AST storyline where it clarifies that Geomancy is a more primitive alternative to Astromancy and ASTs learn from it.

As for the people who mentioned that a non-combo based tank is needed, I think there is a high chance the next tank might be caster-based. The reason is that they opened up the possibility by introducing a casting based rotation for PLD. Therefore, it is possible that we can see more depth to it through the next tank job.