One potential monster girl could be a siren. Her beautiful singing voice could be both a weapon and a way to charm Naruto. A golem - girl made of earth or stone could also be in it. She would be strong and durable, and her powers could be useful in protecting Naruto. Also, a vampire - like monster girl who is attracted to Naruto's life force or chakra could be part of the harem. Her nocturnal abilities would contrast with Naruto's more daytime - active nature.
Well, a lamia - a half - snake half - human monster girl - would be a great addition. She could use her snake - like abilities to help Naruto in stealth or tracking missions. There might also be a kitsune - a multi - tailed fox - girl. Given Naruto's relation to the nine - tailed fox, there could be an interesting connection between them. And a manticore - girl, with her dangerous yet alluring nature, would also be a potential character.
One potential monster girl could be a centaur - like being. With the lower body of a horse and the upper body of a human - like monster. She would be strong and have great stamina for long travels. And then there could be a phoenix - like monster girl. She could have the power of fire and rebirth. Whenever Naruto is in danger, she can use her fire powers to protect him and then rejuvenate herself if she gets injured.
There could be a kitsune - like monster girl, with her fox - like features and tricky nature. Another might be a lamia, half - snake half - human, who could have unique abilities related to snakes, like venom or constriction. And perhaps a mermaid - type monster girl, who could bring in the element of water and her own underwater world lore.
Well, you could have a fox - girl. Given Naruto's connection with the nine - tailed fox, it would be interesting to have a fox - girl in the harem. Maybe she has some of the same powers as the nine - tailed fox but in a more feminine and alluring way.
Maybe a mermaid girl. Since there are water elements in the Harry Potter world as well. She could live in the Black Lake at Hogwarts and help Harry when he has to deal with water - related challenges. Her mermaid magic could be something completely new for Harry to discover.
Well, obviously Fuu would be one of the main members. Sakura could also be part of it as she has always had a connection with Naruto. Ino might be another possibility as she is strong - willed and could be attracted to Naruto's growth. And perhaps some new characters from Fuu's world or other parts of Naruto's travels could also join.
Definitely Hinata. She already has strong feelings for Naruto in the original Naruto series. Sakura could also be part of it as she has a long - standing relationship with Naruto. From other series, if we're doing a crossover, Erza from Fairy Tail could be an interesting addition. She's strong, independent, and would make for an interesting dynamic in a harem situation with Naruto. Also, Rukia from Bleach could be another. She has a cool and collected personality which would contrast with Naruto's more exuberant nature.
Well, obviously Naruto himself would be a main character. His sister could be an original character created just for the fanfiction. In terms of the harem members, Sakura could be one. Even though she has feelings for Sasuke in the original series, in this fanfic she could see Naruto in a new light. Hinata is also a likely candidate as she already has deep feelings for Naruto. Another could be Ino, who might be drawn to Naruto's growth and newfound confidence in this alternate scenario.
Aside from Kushina, Sakura could be a potential member. She might be attracted to Dark Naruto's new - found power and his different personality. Hinata could also be in the harem. Her gentle nature might be drawn to the complex and powerful Dark Naruto. Another could be Temari. Given her own strong - willed nature, she could be interested in Dark Naruto's power and his relationship with Kushina.
Definitely Sasuke. He has a complex relationship with Naruto, and in a gender - bender situation, that relationship could take on a whole new romantic dimension. Sakura could also be in the harem. She has always had strong feelings for Sasuke, but with Naruto's new form, she might develop different feelings. And then there's Hinata, who has always been in love with Naruto, and this gender - bender concept could add more depth to her feelings.
One idea could be that Naruto discovers a hidden realm full of monster girls. They are initially wary of him but as he shows his kind heart and powerful ninja skills, they start to fall for him. For example, a fox - like monster girl might be drawn to his own connection with the nine - tailed fox.