Well, obviously Princess Bubblegum is a strong candidate. She's smart and in charge, and Naruto's power would surely catch her eye. Marceline, the vampire queen, would be drawn to his strength and the mystery around his godlike nature. Then there's Lady Rainicorn, who could be interested in Naruto for his different ways. She might find his ninja skills and godlike powers fascinating, and she could be part of the harem in this fanfiction.
In a crossover with Fairy Tail, Lucy could be part of Naruto's harem. She is strong - willed and might be attracted to Naruto's godlike power and his kind heart. Also, from the Naruto universe, Hinata would surely be in the harem as she has always had feelings for Naruto.
Well, it could be that Naruto, in a strange new world like Adventure Time, has multiple female characters attracted to him because of his godlike powers. Maybe he teams up with some Adventure Time characters and uses his ninja skills in that wacky world while dealing with his new harem.
Hermione Granger would be a likely candidate. She's intelligent, strong - willed and has been Harry's close friend throughout the series. Ginny Weasley is also a possibility. She has a connection with Harry in the original books and her fiery personality could add a lot to the harem dynamic. Luna Lovegood could be another. Her unique view of the world and her kindness would make her an interesting addition to Harry's harem.
Bulma could be one. She already has a long - standing relationship with Goku in the original series. Another could be Sailor Moon. Her strong sense of justice and powerful abilities would make for an interesting addition. Also, Mikasa from Attack on Titan with her combat skills and loyalty could be part of the harem.
In such a fanfiction, Princess Bubblegum from Adventure Time could be in the harem. She's intelligent and powerful, and Naruto's unique skills and personality might attract her. Also, Sakura from Naruto could be part of it. Since she has a history with Naruto and in this crossover, she could be part of the group vying for his attention. And Marceline too. Her cool and edgy nature might be drawn to Naruto's heroic side.
Hinata is a likely candidate. She has always had deep feelings for Naruto. On the island, her gentle and caring nature would make her an important part of the harem. Sakura could also be there. Despite her initial feelings for Sasuke, being stranded on an island with Naruto might make her see him in a new light.
Sakura could be one. She's a strong and intelligent character. Her interest in medicine might make her curious about the blacksmith's skills in creating precise tools. Hinata is also a possibility. Her gentle nature might be drawn to the blacksmith's hardworking and creative side.
Hermione could be a likely character. In the original series, she was always by Harry's side. In this fanfiction, her intelligence could be a great complement to Harry's 'dark godlike' powers. Ginny Weasley is also a possibility, given her connection to Harry in the official story. And Luna Lovegood might be included too, with her unique perspective adding an interesting flavor to the harem dynamic.
Obviously, Kushina herself would be a central character. Minato could also be involved, perhaps in a complex relationship triangle with Naruto and Kushina. Then there are other female ninjas like Tsunade. She might be intrigued by Naruto's time - traveling knowledge and get involved in the story. Sakura could also be a potential character. If she somehow gets dragged into the time travel, she might have her own role in the harem - like situation around Naruto.
There could be some original Arrancar characters created specifically for the fanfiction. For example, a young and curious Arrancar who is new to the ways of her kind. She meets Naruto and is immediately fascinated by his different appearance and abilities. And then there's a more experienced Arrancar who has seen many battles. She initially sees Naruto as a threat but then realizes his good intentions and slowly falls for him, becoming part of the harem.