Rukia could be a character. She might come to Ichigo's rescue when she hears about his situation with the tentacles. She could use her own powers to freeze or cut through the tentacles. Another potential character is Uryu. He could use his Quincy skills to disrupt the energy of the tentacles. And there's also Orihime, who might use her shield - like abilities to protect Ichigo from the tentacles.
Kisuke Urahara might be involved. He has a lot of knowledge and strange inventions. He could be the one who figures out how to counter the tentacles. Yoruichi could also be present. Her speed and combat skills would make her a formidable opponent against the tentacles. And there could be some new, original characters created specifically for this fanfiction, like a mysterious figure who has a special power over the tentacles.
One possible plot could be that Ichigo finds himself in a strange, otherworldly dimension where tentacle - like creatures are a natural part of the ecosystem. At first, he's on guard and tries to fight them off. But as he spends more time there, he realizes they might not be as hostile as he thought and begins to understand their way of life. Eventually, he forms an unlikely alliance with them to face a common threat in that dimension.
Uryu could also be a main character. He might be initially skeptical of Ichigo's transformation into a dragon god but then come to respect and even rely on his new powers in battles against common enemies. His Quincy abilities could contrast with Ichigo's dragon god powers, creating interesting interactions between them.
His friends Orihime and Chad would probably be main characters. Orihime could be very motherly towards baby Ichigo, always making sure he's comfortable. Chad would be the strong protector, making sure no harm comes to the baby.
In the world of Harry Potter, a crossover with Harry could be interesting. Ichigo's immortality would put him in a unique position in the wizarding world. He could bring his combat skills to Hogwarts and perhaps even help in the fight against Voldemort, while also learning about magic and how it differs from his own powers.
Renji could be a key character. He could be a senior student who at first is a bit tough on Ichigo but later becomes a mentor. Orihime could also be there, and she might be in the same class as Ichigo, providing some emotional support and her own unique skills in the academy setting.
There could be Byakuya. He would be shocked and disappointed by female Ichigo's betrayal as he might have seen her as a promising comrade. Also, Rukia. Since she and Ichigo have a special bond, her reaction to Ichigo's betrayal would be very emotional. And then there's Aizen. He could be the one who tempted or manipulated female Ichigo into betraying, seeing her as a useful pawn in his grand schemes.
Rukia could be one of the characters. Since she has a deep connection with Ichigo already in the original series, it would be easy to develop a more romantic aspect in the harem setting. Orihime is also a likely candidate, as she has strong feelings for Ichigo. And from other animes, a character like Saber from 'Fate/stay night' could be interesting. Her noble and powerful nature would contrast with Ichigo's more rough - around - the - edges personality.
Obviously, Ichigo himself is a central character. From Fairy Tail, characters like Lucy could be part of the harem. Her kind and adventurous nature might be attracted to Ichigo's strength and sense of duty. Erza could also be there, with her strong - willed and powerful personality being drawn to Ichigo's unique abilities. And Wendy, given her gentle and healing nature, might also find herself interested in Ichigo.
One possible ally could be a powerful mage from a different anime in the crossover. This mage could help Ichigo understand the strange magic or energy in the new world he is stuck in. Also, Uryu might be an ally. His Quincy abilities could complement Ichigo's powers in Mugetsu form as they face new threats together.
One potential ally could be a friendly youkai student who is intrigued by Ichigo's godlike powers. Maybe someone who is an outcast among the youkai and sees Ichigo as a kindred spirit.