If BL movie follows in usual video game adaptation steps, Lilith will probably get her siren powers at the end when they find the vault and use them for a bit.

That's pretty much exactly what I was thinking tbh. She'll get her siren powers to fight the final boss and her wanting to figure out more about them (as in the games) will be the sequel hook (which may not happen if the movie does as badly as everybody seems to think it will).