In a 'Harry Potter x fem Bowser' fanfiction, it could be that fem Bowser is a part of a prophecy in the Harry Potter world. The prophecy states that a powerful being from another realm will come to either save or destroy the wizarding world. Harry and his friends are initially skeptical but soon realize that she has her own reasons for being there. She could be on a quest in her own world that has some connection to the events in the Harry Potter universe, and together they must figure out how to use her presence to their advantage.
Maybe a new character that is created specifically for this fanfiction. This character could be jealous of the attention fem Bowser is getting from both the wizarding world and Harry Potter's group. They could start causing trouble and trying to turn everyone against fem Bowser. This new antagonist could also have some connection to Bowser's original world, perhaps an enemy from there who has followed her to the Harry Potter universe.
One possible plot could be that fem Lucifer, being intrigued by the magic in the Harry Potter world, decides to visit Hogwarts. She might get involved in the fight against Voldemort, using her own powers which are different from the magic in the wizarding world. This could lead to a lot of interesting interactions between her and the main characters like Harry, Hermione, and Ron.
A plot could be that in a post - war Hogwarts, fem Harry Potter is looking for a new purpose. She discovers a way to cross dimensions and ends up in the world of the Avengers. Clint Barton is intrigued by her and her magic. They form a bond as they try to figure out how to stop a dark force that is threatening both of their worlds. There could be a lot of cultural clashes and misunderstandings at first, but as they get to know each other better, their relationship deepens.
One possible plot could be that the fem Sauron, with her dark powers, infiltrates Hogwarts. She could initially be disguised as a new student or a teacher. Harry Potter senses something is off about her. There could be a series of magical duels and battles of wits between them as Harry tries to expose her true identity while she tries to carry out her own evil plans within the wizarding world.
The plot might involve the fem character being in danger and Fawkes coming to her rescue. Since Fawkes has the power of rebirth, he could sacrifice himself to save her in some way, and then be reborn in a more powerful form. This could then set off a chain of events where she has to figure out how to use Fawkes' new powers to fight against an emerging dark force.
One possible plot could be that fem Arceus somehow ends up in the Harry Potter universe. Maybe she's drawn there by the powerful magic. She could then use her unique abilities to help Harry and his friends in their fight against Voldemort, like using her power to heal the injured or to create barriers against dark magic.
Well, it could be that Fem Kyuubi has some unique powers that are needed to solve a problem in the wizarding world. Harry Potter, being the hero he is, teams up with her. They start by trying to figure out how her powers can be used to stop a dark force that is starting to spread in Hogwarts. Along the way, they learn about each other's cultures and form a deep friendship. For example, Fem Kyuubi could teach Harry about some ancient magic from her world, and Harry could introduce her to things like Quidditch.
One possible plot could be that Scooby Doo and the gang are on vacation and accidentally stumble into a magical portal that leads them to Hogwarts. The fem Harry Potter helps them adjust to the wizarding world while they all try to figure out how to get the gang back home. Along the way, they discover a mystery involving a missing magical artifact and have to work together to solve it.
A possible plot is that there's a magical experiment gone wrong at Hogwarts that creates a portal to Danny Phantom's world. A female Danny Phantom enters the Harry Potter world through this portal. The Ministry of Magic thinks she's a threat at first, but she proves herself useful when a new dark force emerges. She teams up with Harry and his friends, using her intangibility and other powers to their advantage in battles and sneaking around the castle.
Well, perhaps the female Harry Potter uses her magic skills in the world of being a hitman. She could be using spells to outwit her targets or to make her escapes. Maybe she has a special wand that she uses as a weapon in a more non - traditional hitman way.