Well, perhaps she could go to Vegas to escape the wizarding world for a while. Maybe she'd get involved in some sort of magic - related mystery there, like a dark wizard using the glitz and glamour of Vegas to hide their evil deeds.
She might meet a powerful and mysterious wizard who has been hiding in Vegas for a long time. This wizard could be either a potential ally or an enemy. There could also be a group of young witches and wizards who are having their own adventures in Vegas and they all team up with fem Harry Potter for a wild ride full of magic, mystery, and fun.
In a Vegas - set Harry Potter and Supergirl fanfiction, it could be a story of a magical tournament held in Vegas that attracts wizards from all over, including Harry Potter. Supergirl, intrigued by the strange magical happenings, investigates. They form an unlikely friendship as they both try to keep the peace during the tournament which is being targeted by some anti - magic groups who want to expose and destroy the wizarding community.
He could marry a Muggle he meets by chance in Vegas. Maybe a singer or a waitress who has a big heart and an open mind towards the strange things Harry might do or say because of his wizarding background.
Well, Harry could be really confused at first. Ouran is a very different world from Hogwarts. He might be shocked by the opulence of Ouran Academy and the strange club system there. Maybe he'd try to find a way to fit in while still being true to his wizard self.
One interesting story could be about Harry and his friends visiting Vegas during a wizarding world vacation. Maybe they get into some magical misadventures in the Vegas casinos, like accidentally turning the slot machines into magical creatures. Another could be a story where a Vegas - based wizard tries to bring some of the glitz and glamour of Vegas to Hogwarts.
Maybe Harry Potter uses his magic to create some amazing illusions on the Vegas Strip, and Supergirl is both amazed and a bit concerned about the chaos it might cause. She helps him keep things under control while also having fun with the new magical experiences.
In some versions of this fanfiction, the Vegas part could be a parallel dimension that Harry accidentally stumbles into. There, he discovers a different set of rules and meets a person who is his soulmate in that dimension. Since he has always been a bit of a rule - breaker and a hero, he decides to marry this person despite the differences. It could also be that the story is a comical take on Harry's life, where he goes to Vegas with Ron and Hermione for a lark, and things go hilariously wrong and right, resulting in him getting married.
It could be a story where Harry and Hermione decide to elope to Vegas and have a magical wedding. They might find a wizard - friendly chapel that has spells for eternal love and protection. The Weasleys could show up in a flying car to surprise them, and there would be a lot of magical chaos and celebration.
It could be a powerful dark wizard who has escaped from Azkaban and has made Vegas his new lair. He's using the chaos and glamour of Vegas to hide his dark magic activities, like creating dark spells and recruiting followers. Harry would have to face this wizard to protect both the wizarding and Muggle worlds.