So, I might be canonblind to Persona 5, but from what I know about Haru, I feel like she and Athrun would be interesting to play off each other. They both have issues with their fathers putting their personal goals over the well-being of their kids, and they both come from wealthy families. The main difference is that Athrun is used to everyday society outside the bubble of upper-class lifestyles and leans toward the cynical.

As for PSL ideas, I have a fair few generic ones, like arranged marriage or Pokémon AU. I could even AU Athrun into the Persona world.