It could be Harry Potter himself. Imagine Harry being sent to the Star Wars universe. His heroic nature and powerful magic would attract various characters. Maybe Rey from Star Wars would be interested in his magic and his ability to stand up against dark forces. And some female Hogwarts students might also be part of his 'harem' as they continue to support him in his new adventures.
A new character created specifically for this fanfic could be the main character. This character could be a hybrid of a wizard and someone with a connection to the Force. They would grow up in a world where both magic and the Force are known, and as they journey through both the Harry Potter and Star Wars universes, they would attract a diverse group of characters from both, forming a harem that is based on mutual respect and a shared goal of protecting both universes.
Loki from Marvel could be an interesting main character. He has his own charm and mischievous nature. If he somehow gets involved with the Harry Potter world, his wits and powers could attract characters from both universes, creating a harem around him.
A young half - blood wizard could be a great main character. They might have grown up in a muggle neighborhood but always felt a connection to the magical world. Once at Hogwarts, their ability to empathize with magical creatures makes them stand out, and gradually they attract a harem of creatures.
Peter Parker (Spider - Man) could also be a great main character for this kind of fanfiction. His youthful charm and heroic nature would appeal to characters in both universes. He could be on a school exchange program in Hogwarts (using some multiverse - like excuse) and during his stay, he gets involved with helping the wizards while also catching the eyes of Ginny Weasley, MJ, and Black Widow, creating an interesting harem situation.
The main character could be a padawan who was orphaned during a Sith attack. This padawan has always had a strong connection to the light side but has been struggling to fully embrace it due to the trauma. Through the story, they learn to 'let the light in' and become a powerful Jedi.
Darth Vader could be the main character in a second - chance fanfic. He could be brought back to life and given the chance to fully embrace the light side from the start. He could use his knowledge of the dark side to help the Jedi fight against a new and powerful evil in the galaxy.
Another option could be a hybrid character. Maybe a half - Dark Angel, half - Star Wars species character. This character would be constantly torn between the two sides of their heritage, having to decide whether to embrace their Dark Angel side, which might be more about physical strength and survival, or their Star Wars side, which could involve the moral choices related to the Force.
Palpatine could be made into an ancient evil that has been dormant in the Harry Potter universe for centuries. When he awakens, he starts to spread his influence, using his political manipulation skills to gain power within the wizarding world. He could form alliances with dark wizards and creatures, much like he did with the Trade Federation and other groups in Star Wars.
Obviously, Harry Potter would be one of the main characters. Then, there could be a lead cowgirl, say, a strong - willed girl named Daisy. She could be the one who first befriends Harry and brings the rest of the cowgirls into his world. There might also be a wise old cowgirl, like Grandma Willow, who provides guidance and knowledge about their unique magic.
A Mandalorian could be the main character. Mandalorians have a rich culture and history in Star Wars. A story about a Mandalorian who has to break from their traditional ways to save the galaxy from a new threat, like a powerful Sith Lord or a dangerous alien race, would be exciting. They could use their unique combat skills and armor to fight against the odds.
Another possibility is Hermione Granger. She is intelligent, brave, and resourceful. In this type of fanfiction, she could gather a group of interesting characters around her. Maybe she could have relationships with characters like Tyrion Lannister for his wit and Ned Stark for his honor. This would create a very different dynamic compared to if a male character was the main one, and would explore different aspects of the harem concept in this crossover.