Honestly, kind of is too early to speculate, but Emet-Selch did kind of vocalize protentional expansions for the future, ie Additional areas in our classic zones, the other continents, and the remaining Shards "You'll be surprised what you find there." This said, what I want... Hammers, Warhammers, I want a Job that uses Warhammers, Big Honking Chunk of flat metal, designed to Pound enemies into submission. So I guess going with that theme, and looking at past FF games...
FF1 White Mage & Warrior
FFIII Viking & Onion Knight
FFV Freelancer & Berserkers
FFXI White Mages & Geomancers
((Note there are other hammers in other games, but these are associated with characters and not jobs/classes))
Looking at the list, we can break Hammer use into several classes. Viking, Onion Knight, and Freelancer can be covered by other jobs and classes. With Onion Knight and Freelancer being no Job Adventurers, and Viking is either a Warrior or Berserker.
White Mage, Geomancer - White Mage is our basic Healer Job, Geomancer is essentially the DPS version of a White Mage. This would actually be an interesting Job, and because at high levels White Mages are using more White Magic and less Earth Magic I would say that Geomancer is a possibility.
Warrior, Berserker - Warrior is one of 2 of our Basic Tank Jobs, and Berserker is essentially the DPS version of a Warrior, like Geomancer there is room for it in FFXIV, however I do not see a lot of evidence they will include it.
What I suspect, Geomancer Physical DPS (Uses Ninja Gear) like Ninja it has a mix of magical abilities and melee abilities, Gaia will be the Job lead.
--Edit, long reasoning behind Geomancer as physical DPS and not Magic DPS.
Geomancer What it does in past Final Fantasy games. Basically it's a white mage, the spell lists, and even the gameplay is a DPS White Mage, that brings nothing new to FFXIV, but then if we look at how the team made other Jobs in FFXIV we see a trend of tanking the basic concept (Sage, Red Mage, Gunblade, Dark Knight, Dragoon, Summoner) and making it unique and it's own thing. So how could they bring Geomancer to the game and make it fun and unique without taking away from the unique gameplay of other jobs. By doing something New. Melee Caster, ie looking at the trope of the Earth Mage from popular fiction, we see that a Geomancer could easily fit in a Melee role, using Earth Magic for hard hitting Near Ranged and Melee Combos. Kind of like how a Ninja uses both magic and melee. And since Ninja is the only Job that currently shares gear with no one else. I would say the Next Job should go here. So looking around at Dex based Melee, with hammers and casting magic who do we see, Gaia. Our one and only Goth Cutie, on the first. Her BFF (and possibly Romanic partner is a rogue.) She is very clearly both Earth Magic and a Dex based Warhammer user.
I would make the Job a two handed Big Hammer, and basically the opposite of Red Mage, it does it's main combos and build ups in Melee, then steps back to hit them with Big Hits of Earth Magic. Which once again is kind of like Ninja, only with a very different type of weapon... Oh and here is an anime list of Earth Casters to get a sense of aesthetics.
https://www.cbr.com/anime-characters-best-earth-powers/