So as far as the Persona 5 Royal teaser, we don't actually know for sure that's a female Joker. She says something along the lines of, "I don't support the Phantom Thieves" (this is what I've heard, I don't know Japanese) which would be strange for Joker to say. Fans on the Persona 5 board at GameFAQs are speculating she could be a new confidant or antagonist. She does have a ponytail, though, which lines up with the datamine. But it could be just a coincidence.
We won't really know until we get more info. Personally, my heart skipped a beat when I opened this thread and saw "Female Joker confirmed," so yeah, I really, really want it to be the case. Damn it, Atlus, don't play with me like this.