Noctis is going to be an older brother.

Preferably Ignis, but Prompto or Gladiolus are fine too. King Regis manages to knock up one of Noct's friends, through a mutual stress relief or secret relationship, whatever. Regis can know or he can die before he finds out. I/P/G + Noct's reaction when it happens especially it he didn't know about the relationship with his father.

Or just if Regis/I/P/G has an impregnation kink that's cool tool.