I think it would be a part of some type of vision where Rey would be presented as Kylo - notice that she is dressed like him and her saber is crackling, just like Kylo’s.

Of course there is a chance of Rey turning to the dark side, but I doubt JJ and the others would reveal such a major plot twist in a trailer, and not even a final trailer. So it’s just a very interesting, but not very real tease of Rey going to the dark side, reminiscent of trailers for “The Last Jedi” where we see Kylo holding his hand to Rey.