Geomancer

[ArcaneCarbuncle]

I think they only use those three elements because at some point they were trying very hard to set Onymuji and Geomancer apart (I assume to parallel CNJ and THM). But at this point the line that separates the two seems to fluctuate. They could just have the two cannibalize eachother and just be one straight up eastern entity though and give it to the playerbase. My personal hope is that whatever the playstyle ends up being, I really hope through the use of certain spells/oGCDs they end up able to place fields at their feet that end up having different effects. This way we would still get the feel of terrain manipulation but don't have to place targeted aoe's. (since they seem to not want dps to use those, thank god). And from what they have shown us of the class so far, they aren’t interested in the “all elements” theme. They seem pretty honed in on the name of the class and went full on Feng Shui.

The thing is that all they've shown so far is two years or older at this point and was created when they likely weren't planning on making GEO an actual job.
Should they have started/are starting to look into adding GEO as standalone job plenty of things are likely to have changed.
Kyokuho for instance is at best equivalent to what 3.4 Alisaie was to RDM (i.e., a stand-in that has nothing to do with how the job acually plays), and the Geomancer Mobs in Swallows Compass/HoH dont have to mean all that much either seeing as they don't use standard player models/animations. SE could base GEO on them, they could however as easily go the route of the last boss of Amdapor Hard, which teased a bunch of then suspected future WHM Abilities (including Cure 4!) - except none of those abilities ended up being added.
Ultimately I see it this way: Astrologian has been in the game since ARR and the AST we got was vastly different, Gunblades have also been in the game since ARR and Gunbreaker is very different to everything we had seen involving Gunblades before; I'm for one am not comfortable basing my expectations of GEO on stuff that was released over two years ago.
Nor do I want to tbh, and I repeat myself: I'd want them to come up with an interesting, unique concept first and then adjust lore from there, rather then limiting what they can do with the job because lore; and I say this as a lore-aficionado. Obviously I would't want them to outright break anything, I just dont want them to stifle their creativity.

As for the Geo and Ast thing I was commenting on how you said SE was shoehorning Geo into Ast, which I didn't really see. But I do think they probably saw player feedback and really tried to separate the two!

Shoehorning might have been a strong word, but it very much still seems to me like they planned to have AST and GEO linked together rather then create a standalone Geomancer job.

Earthly Star is by no means the first targeted ground ability in the game. I think the tooltip is even something to the effect of you are placing a celestial body on the ground. So while I agree that terrain has played a big role in Geo as a whole, I don't think the placing of Earthly Star was a branch out experiment into the class. Not when we have Asylum, Sacred Soil, HAD Shadowflare and I think Drk gets one as well. Ast just needed a placed aoe healer circle to match the other two healers.

I might not have explained it well, but I never meant to say it was the first ground-targeted AoE, I meant to point out that among that group of abilities it was the most elaborate/unique.
All other similar Abilities simply apply their effect over time to anyone standing within, whereas Earthly Star basically sits one the field doing nothing until you pop it. And then it affects both enemy and ally in different ways.
Of course its possible they made it different just for the sake of being different, but at the same time they could have planned to do more with it at some point in time.
You know, like creating different types of temporary environments to be used by a job that historically used their environment for different effects. Just saying.