Personally I don't see what she does as "against everything we know with the monk". She uses her fists (like the monk) and legs (like the monk). The trailer show that she does have more fluid motions and uses her cloth while fighting. That to me is the only difference. But it could also be just something they added for the visual effect of the trailer. It's not like there has never been a trailer that show stuff that actually never appear ingame.
Also, "add a weapon"? If you're refering to the pictures where the girl have a blade attached to her arm, it's not like the Monk never had any blade weapons before. And that it's a hidden blade, remember the Sephirot Monk weapon? The one that looks like a glove but actually isn't a glove, and (only a small part tho) is hidden when not being used.
But like I said, no one is right or wrong at this point. I just personally don't see those 2 people in the trailer to be of different classes.
If it would turn out that she is a Dancer, well good for those who want a Dancer class. But when I'm watching the trailer and think that she's a Dancer I get kinda turned off. Because the Monk and the Dancer looks way to similare in the trailer then. Both use stances and fight with fists/legs. I thought SE wanted to make each class/job feel unique and special (as much as possible atleast).
Edit:
Too me that looks pretty mirrored (matching fighting style). Only difference is the color of the auras.
There is also that time when they both hit/block each others kick (could not find a picture).
It could just be a coincidence, maybe not. Only the future will tell.