Ooh! How about Fantastic Beasts in The Phantom of the Opera AU?
Albus is Newt and Theseus's uncle. After the brothers' parents died, he took them in and raised them. They lived in an opera house and still do except Theseus, who moved out.
I think Leta would make an interesting Christine. She was broken, and The Phantom was, too. I think that would make her even more likely to sympathize with him, fall for his manipulation, and feel confused.
Newt is Leta's best friend since childhood. He's studying to be a zoologist but earns money working backstage in the opera house. He's got a lovely singing voice, but he isn't into performing. However, he's comfortable singing to Albus, Theseus, and Leta. He sings to and with Leta during the "Angel of Music" song.
Theseus met Leta after she moved into the opera house. He was polite to her as she was just Newt's best friend to him, but after some time, when they grew to become more comfortable with each other, he became a brotherly figure to her. He's four years older than her and Newt, who are 20. He works as a police officer.
Grindelwald is The Phantom. He's 10 years older than Leta.
Leta's backstory is a combination of what it was in Fantastic Beasts and what Christine's is in The Phantom of the Opera but with a few changes. Her father, who had never loved her, died when she was a younger teenager. She was then sent to live with her uncle, who did like her and treat her well. She loved to sing and wanted to be an opera singer when she grew up. He supported that. When Leta was 16, her uncle got deathly ill. He told her that after he died, he would search for the Angel of Music and send it to her. After he passed, Newt told Albus and asked him to take her in, knowing the alternative was the foster system. And so Albus did.
Years before that, Albus saw a homeless Gellert Grindelwald in a cage and getting bullied for having a disfigured face. Albus was disgusted by the bullies' behavior. He saved Grindelwald from them and let Grindelwald live in a lair in the opera house. He didn't tell anyone this though.
Sometime after Leta moved in, she went to a room to light a candle for her late uncle and brother. She then heard a voice speak to her. She didn't see anyone, so she thought the voice belonged to the Angel of Music.
They talked, and she told him of her desire to be an opera singer. Grindelwald had learned singing techniques from watching and listening to performers in the opera house while he had been living there, so when Leta referred to him as the Angel of Music, he pretended that he was. He then began to give her singing lessons regularly.
In the present day, Grindelwald, whom Leta also refers to as The Phantom of the Opera, makes written demands to directors to give Leta lead roles in operas. If they don't do it, he kills them. He also falls in love with Leta and tries to get her to choose him. He's very toxic about it though. His behavior alarms people, including Leta.
Albus confesses to Theseus, who's trying to handle the situation as an officer, that he let Grindelwald move in years ago and why. Albus doesn't agree with what Grindelwald does, but he still sympathizes with Grindelwald. He sees Grindelwald as a misunderstood man that was oppressed and hurt from it. Theseus gently tells him that Grindelwald has taken things too far.
Everything else goes according to the story except that she refers to her uncle, not her father, in the song "Wishing You Were Somehow Here Again", and after The Phantom abducts Leta and brings her to his lair, Theseus arrests The Phantom. She looks at The Phantom with pity as Theseus takes him away.
Newt comforts Leta as another officer drives the police car containing Grindelwald away. Theseus gets rid of the crowd and then joins the two after that. Albus comes, looks at the three of them sadly, and then leads them to the rooftop garden, where they can be alone.
Leta tells them she feels bad that The Phantom went through so much suffering. She thinks she deserves to be hated because she's a monster that "killed" her brother, but The Phantom was hated for an unjustified reason and that he became a monster. Newt is very tender with her and agrees that what happened to The Phantom is sad and that he didn't deserve that, but it didn't give him the right to do any of the awful things he did. Theseus agrees with Newt, and Albus says that he made excuses for Grindelwald out of sympathy, but that he shouldn't have. Cause Newt and Theseus are right; Grindelwald shouldn't have committed the monstrosities that he did. Leta sighs sadly but nods in understanding. The three men also comfort her about her past and tell her that Corvus's death was not her fault. All four of them sing the song "All I Ask of You" to each other, except that the song is changed so that the romantic elements are taken out and the song is about comfort and support from them, her family.
Eventually, they all move on and live the rest of their lives happily.
