I'm thinking of doing this. This was inspired by a fanfic I read on here and I asked the author if I could use some of her ideas and she said yes. It's a religious AU that takes place in apocalyptic world between the natural and supernatural forces.
In 1462 two warriors known as Cat Noir and Carapace were allies and knights of the church who fought in a war between Muslim Turks. After the war there was tragic misunderstanding that led to their friendship turning into a bitter hatred and they were transformed into the two most hated, feared, and evil creatures in all creation.
Centuries later It's World War 3 and humanity is fighting against devils and vampires for control over the earth. There are two forces that fight against these forces, The European division of Exorcists and the Asian division of vampire slayers.
Marinette Dupain-Cheng is a nun of France. Her guardian Master Fu and the head of the Miraculous Church of Exorcists, disappears one day leaving behind a letter saying that she is now the lady and mistress of the church. Confused she writes to him asking why and he responds that the answers lie in the tomb of Christine De Lacy, the heavenly woman who founded the church. She goes to the tomb and accidentally releases the long imprisoned devil Cat Noir. Cat Noir tells her that she is the reincarnation of Christine and that he was his mission two hundred years ago to protect her and now he must protect Marinette. He becomes the church's secret weapon against demons who disguised as a human ally from England known as Adrien Agreste.
In the Middle East Alya Ceasire's father is the head of an organization of vampire hunters. When he dies she's left as his heir but her father's partner who's been after control of it for years, angrily tries to kill her. As she tries to escape him she cuts herself and stumbles on to a seemingly dead corpse who starts speaking to her. He begs her to give him just a drop of her blood. She does and the corpse is revived to the vampire Carapace who kills her father's partner and tries to scare her until she bravely stands up to him. Seeing her temper he recognizes her as the great granddaughter of the man who staked him years ago. Impressed by her courage and grateful for the blood she gave him he offers himself to her as a servant. She accepts and like Adrien he disguises himself as a human ally from Romania under the name Nino Lahiffe.
These two girls with use their protector/servant to help them slay the vampires and devils and defend mankind. Eventually Marinette and Alya meet and decided that they should join forces but their "Non human boyfriends" are still bitter toward each other.