Obviously, Harry Potter himself would be the central character. Hermione could be part of the harem, as she has always been close to Harry. Luna Lovegood might also be there, with her unique and dreamy personality. Additionally, some new original characters could be introduced, like a powerful witch from a Vegas - based wizarding family.
Well, perhaps in the fanfiction, Harry Potter is on a mission in Vegas. Due to some magical mishap, he attracts a harem of characters. Some of the plot could revolve around how he manages to keep his identity a secret while also dealing with the complex relationships within the harem. They could be helping him solve a mystery that is tied to both the wizarding world and the unique Vegas magic.
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.
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.
Perhaps in this 'Harry Potter in Vegas' fanfiction, Harry accidentally apparates to Vegas instead of his intended destination. There he meets a group of American wizards who are using Muggle Vegas as a cover for a secret magic convention. Harry gets involved in a mystery where a powerful magical artifact has gone missing and he has to find it before it falls into the wrong hands.