Dancer has been a job in the FF series since FFV if I recall. Dancer can be implemented in many a variant of any of it's prior implementations from the series (or have a new spin of it's own for XIV).
For example, in FFXI (I mained DNC and Red Mage) Dancers were a sort of frontline Damage Dealer and Support/Healer that strictly used TP. Dancer's deal quite a good amount of damage and used TP to heal/buff themselves and the party and debuff enemies through dances (Curing Waltz, Drain Samba, Haste Samba, Quick Step, Wild Flourish, etc.). They mainly used Daggers, and eventually got native dual wield like Ninja had. If they sort of base DNC like FFXI's it could easily be a DPS with support abilities similar to how Bard is in this game (DPS with MP/TP refresh and magic defense debuff as songs. Dancer could have like Drain Samba that acts like a weaker Bloodbath, Aspir Samba to help refresh the Tank and Bard MP, and another one that isn't Haste Samba because the devs don't want to give us haste).
Or they could take a note from other games, like FF Tactics Dancer was more strictly of the support role of the debuff sort. If you did one of their dances they would continue the dance until you say to stop, each dance had it's own effect as well (one did damage, another drained enemy MP, I think one inflected Toad, etc.). They could take this concept and make Dancer like a healer. Say it has regular cures (Curing Waltz) and can do an AoE HoT that works similar to bard songs (drains MP, or TP if cures use MP).
There are a lot of possibilities and ways to implement the job just looking at the series history.
Also one does not merely stand still while dancing, dancing keeps you mobile right? I mean if a Bard can sing while dodging Landslides, I'm pretty sure a Dancer can tango around Titan's attacks. Hell, Black Mages and White Mages need to stand perfectly still to cast their magic, and you're telling me a Dancer wouldn't be able to dodge while dancing? In FF Tactics Dancers could move around while dancing (it was persistent until you canceled it), and dances weren't always a persistent ability in the job's many iterations. Just throwing that out there. :P
EDIT: And the least likely job(s) to make an appearance in this game, in my opinion, is Onion Knight and Freelancer. Never gonna happen.