It could be a new character that is a combination of a dark wizard and a Hollow. This creature would be able to use both dark magic and Hollow abilities. It would target both Hogwarts students and Soul Reapers. Ichigo and Harry would have to figure out its weaknesses, which would be a difficult task as it has powers from two very different universes.
It could be a dark wizard who has somehow managed to gain the power of Hollows. This antagonist would be extremely dangerous as they would combine the dark magic of the wizarding world with the destructive power of the Hollows. They might be after the cousin because of some special power or connection that the cousin has, which could be used to either rule both worlds or destroy them.
Fem Ichigo could interact with the Harry Potter characters in a very unique way. Given her own background, she might approach them with a mix of admiration and her own sense of self - confidence. For instance, she might offer to train Harry and his friends in some form of combat, something that could be useful in their battles. She could also share stories from her own world, which would be really fascinating for the Hogwarts students. And in return, she would listen intently to their tales of magic and mystery at Hogwarts.
Definitely Hermione. She is smart and would be able to help Ichigo understand the complex magic world. Also, Hagrid. He is kind - hearted and has a lot of knowledge about magical creatures, which could be useful for Ichigo.
It could be a new character created by the fanfiction author. Maybe a powerful wizard who was once shunned by the wizarding community and now wants revenge by stealing magic.
A dark wizard who has merged the dark arts with Wonderland's magic could be the main villain. He might have created some sort of dark Wonderland where all the creatures are under his control, and Harry has to break this control to restore balance.
When writing a fem Harry Potter Ichigo fanfiction, it's important to blend the two worlds seamlessly. You could start by mapping out how the different power systems will work together. For example, how would Harry Potter's magic interact with Ichigo's Hollow powers? Develop the characters well, especially the fem Harry Potter. Give her unique traits that are both true to the original Harry Potter character and also influenced by the new situation of being in a crossover. And don't forget to have a strong plot that drives the story forward, perhaps a quest to save both worlds from a common threat.
First, you need to understand the basic elements of both universes. Know the rules of the 'Bleach' world regarding Soul Reapers and Hollows, and also the magic system in Harry Potter. Then, think of a unique concept for your story. For example, what if Ichigo was a wizard in Hogwarts? Next, focus on character development. How would Ichigo and Rukia interact with the Potter characters? Finally, just start writing and let your creativity flow. Don't worry too much about making it perfect at first.
Maybe a new character, a Muggle - born wizard who was shunned by both the Muggle and wizarding worlds. This character, out of bitterness, decides to combine Muggle technology and magic to create the droid army and rule over both worlds. He would see Harry Potter and his friends as the main obstacles to his grand plan.
It could also be a dark wizard from the past who has been trapped in a hidden part of Hogwarts and is now released during Harry's third year. This wizard could have a vendetta against the Potter family or Gryffindor house and start causing trouble.
It could be a Death Eater who has somehow made a pact with a powerful force from Scorpian's world. This villain would be extremely dangerous as they'd combine the dark magic of the Harry Potter universe with the unknown and potentially deadly powers from Scorpian's realm.