Geomancer is always a possibility for a healer since it *has* always had at least one "heal" as part of its Geomancy repertoire. A lot of it depends upon what they base Geomancer off of. A Geomancer grown out of CNJ is most definitely *not* going to be a healer and is, in my opinion, most likely going to be a CNJ DPS class. A Geomancer grown out something like MAR (in Tactics, they used Swords/Axes and were a balanced physical class) would also be a DPS class (in that case, they would likely have "support" as their secondary role and utilitize Geomancy in the same way that BRDs use their job abilities). A Geomancer grown out of THM could be a possibility for a healer, though the fundamental THM mechanics would present some design problems since you can't really have a healer that uses the burn/regen phase set up that THM uses (it would most likely have to completely repurpose Astral Fire/Umbral Ice, likely into something that changes the secondary/bonus effects of it heals between shields and HoTs, and repurposes some other aspects of the class to boot). I'll agree that it's not *likely* to see Geomancer as a healer, but it's not outside the realm of possibilities; I find it much more likely that it'll be a DPS unless its given an entirely new class to be grown out of, which I'm not entirely sure is going to happen.
Sage isn't likely to crop up since we've got scholar and the two are often conflated as the same class, specializing in casting bother white and black magic while being a full blown caster. SCH's schtick has regularly been some variation on "Scan", which we didn't get in this game (for good reasons, honestly, since I don't really see if being of any real use), so I doubt they'll add Sage since, really, it would be the same thing.
Chemist is also unlikely since we've got Alchemist as a DoH class. There are Chemist-healers in the game as NPCs, but I'm pretty confident that they really just have renamed CNJ/SCH heals and were only given them to avoid being completely and utterly useless as helper-NPCs. On top of that, Chemist always used consumables as its mechanic and the devs have outright stated that they don't want any classes based upon consumables in ARR.
Seer and Oracle are both wonky since they don't really have much to define them yet so they're really kind of wildcard-ish. The only game where Seer isn't simply an upgraded WHM (FFT:A2) has them existing as a weird BLM/WHM fusion class with some personal resource utility that only gets access to higher level magic (and, even then, mooches it from other classes). Oracle only exists as either a magic roulette class (abilities with a random spell list) in the remakes of FFV or a debuff specialist in FFT that required WHM to unlock it. Both of suggest some degree of White Magitude but neither really has a particularly strong presence in the FF series. If they were brought in, I expect them to be based off of a completely new class that provides some alternate healing schtick, likely a support/healer akin to SCH.
The most likely new healer, in my opinion, is dancer: it has a strong presence in the FF series, a well known skillset based around providing team support rather than outright damage, and has an easily accessed gimmick in being a TP/MP resource-fusion healer grown out of a DoW class (likely Thief, as I see it though ARC is a possibility if they could get around the problems of ARC not having anything that could really be repurposed as a heal).
Part of the problem with coming up with new healers is really identifying a new mechanical niche for them to fill: if they try to fill a niche that's already taken by an existing healer, you have to ask yourself why bother unless it's sufficiently different in some other way that isn't just thematic/aesthetic (like a completely different resource model akin to my TP/MP idea for DNC). WHM already covers most of the "HoT-healer" archetype and SCH covers both "shield-healer" and "pet-healer". Some of the only other major healer remaining archetypes that I can think of are the "puddle-healer" (throws down persistent ground effects that provide healing; the most likely implementation for Geomancer-as-healer) and the "aura-healer" (creates persistent/long term area effects around the caster or target that move with them; DNC could possibly use this but I'm not fond of the idea) since straight up direct healing is something that all healers have as baseline functionality. Another *possible* archetype would be the "retributive healer" that provides buffs that heal the target whenever they take damage with multiple charges or medium length durations though this creates problems in healing anyone that isn't a tank so they'd likely just have to rely upon a generic AoE heal.