I gotta agree with you. At this point my hopes/thoughts are pretty similar to yours. Considering the Smash datamine, I definitely think she'll be a "FEMC", as in a female persona user that we play as to the same extent as Joker in the base game. However, I don't think she's just a female version of the same character.

I think it'd be really interesting to see them do something similar to what you said,. Like a lot of other people pointed out, she clearly doesn't support the Phantom Thieves, at least in the beginning and that could make her a great foil to the Thieves and/or Akechi. I'd love to have her story happen at the same time as the main plot of the game, and maybe continue afterward with her trying to figure out the Theives, or explore metaverse alone.

I can't see her being related to Haru, who pretty much loses all importance to the story after the incident with her father. My bet is that Royal is something symbolic that ties into her Persona or character arc.

It also wouldn't surprise me if she winds up interacting with Akechi in some way. Considering his popularity in Japan, and the development he got in outside material like the anime or Q2, he's got to have more of a role in Royal. If not blood siblings, they might be associated in other ways; maybe she's a fan of him early on, or she winds up helping him "solve" cases. However they do it, it'd give him some character development by giving him someone unrelated to the Thieves, or the Conspiracy, or his job to talk to. I'd love to see some conversations where they compare his "justice" with her actual justice.

If she is allies with Akechi, I think it'd be neat if she joined the Thieves in Sae's Palace - she could be a replacement Bless- user after he leaves the party. They could join the party together, but she'd split off from him after AKechi pulls his stunt at the end of the palace. She could find out that he's a murderer/fraud, causing her to fully join the party for the last 2 palaces. Maybe they could have your decisions during her route or let her be the character that decides whether he deserves a shot at redemption or not.

I don't think the trailer location is important. It's just the center of Shibuya - a lot happens there in the game, but I don't think it ties into her joining the Thieves. The trailer actually reminded me of this scene from the beginning of the game where Joker finds the Nav app on his phone.