Yda is a pugilist, she's been referred to as that in the story.
Papalymo is referred to as a thaumaturge.
Thancred first calls himself a bard. Even though he, up to 2.2 quite obviously used gladiator skills, now he's been called a rogue and looks like one when he draws his weapons.
Y'shtola has been referred to as a Conjurer, though she's quite often uses Collective Unconscious, which is an AST skill.
Milfilia had a single dagger, though she was never shown in combat so it's likely that she had no 'battle' class/job.
Urianger is an arcanist, however he wields a staff which is not possible by player arcanists, (though in the 3.4 story during his costume change he uses a book).
Alphinaud is an Arcanist, and as such wields a book and summon carbuncles of different colors.
Raubhan is a gladiator, and as I said in my post, used to dual wield swords. Though player gladiators are relegated to a single sword and shield.
That's just the scions and Raubhan. Most have classes, and use skills/weapons outside of what a player character can. So, by looking at the teaser trailer, with the fact that this mystery girl meets the WoL who has 100% been confirmed to be a monk with nearly the same fighting style, one can safely assume that she is a monk or a pugilist. NOT a dancer.
If it's revealed later that she is one, it'll be a first. Because up to this point, they have not showcased a single new job in any trailer. Plus, if dancer was the case, wouldn't have been more wise for Yoshi-P to have worn say a "Dance dance revolution" shirt instead of scarlet witch if they wanted to hint at that job?
Instead, they gave us a taste of the expansion, Ala Mhigo, the hand of Rhalgr (and the new hub) and two sparing monks. People are really grasping at straws and doing mental gymnastics to force a narative that isn't there. If Dancer is a job that we get with Stormblood, great! However, at this time there is absolutely nothing that we've been shown that even hints at it.