Geomancer

[ArcaneCarbuncle]

I don't think anybody was saying other elements are completely off the table – with the chambers of Swallow's Compass ending in "Heart of the Dragon" with a fire-throwing tengu inside, I could see a rotation built around cycling between Earth, Wind and Water to end with a massive Fire finisher (perhaps magma-based to maintain distinction from the fire spells flung by other casters), much as BLM and RDM build up to non-elemental finishers. Plus it could tie in well with the classical elements, even though Feng Shui uses the five Chinese elements (excluding wind, and including wood which has the same place, and metal).

If you mean all six though, I should point out none of the dedicated casters practice more than four elements – largely because more than three isn't really necessary for all practical purposes (hence why SMN has only 3 Egis despite six pure-elemental primals), and RDM's dual model essentially gives it clones of really only three spells. So while Ice and Lightning aren't off the table, they wouldn't entirely be unnecessary.

Well, there is so much discussion going on about GEO being oh-so-similar to Conjurer that it seems to me that people are forgetting that the other elements were part of the classic skillset as well. So I thought I'd put it back on the table.
Real-Life Feng Shui doesn't need to have much bearing on how GEO would be designed here, seeing as in-game it seems all but accepted anywhere that there are six elements; and other games' iterations of the job also mainly focus on the whole terrain/weather-gimmick which had nothing to do with Real-Life Feng-Shui either.
I also feel it's important to point out that AST for instance, while being based on RL-Astrology, takes plenty of liberties with the concept, so GEO can take as many liberties with Feng-Shui as it needs.
In many ways, I find this games elemental cycle(s) already reminicent of the Feng-Shui elements, but maybe thats just me.

Mechanically there are many ways to include all elements, they could for instance create a dual-rotation - one on the GCD, one on the oGCD - based on the two elemental cycles we've learned about.

Ultimately though I think a full elemental caster is a concept that they haven't explored before (other than Y'shtolas Sorceress), and might be an interesting concept for them to take given GEO's history.
And I think they should look at the entire History of the job should they ever choose to create it, not just the bits and pieces of lore they created when they still tried to shoehorn GEO into AST.
Ignoring an otherwise interesting concept because 'practicality' also seems like a waste, considering the elements are purely visual in this game anyway.