Kakashi can be a popular character too. His wisdom and combat skills can be useful in fanfic scenarios involving Kaguya. He could be a mentor figure to Naruto, guiding him in dealing with Kaguya - related threats or helping to decipher ancient ninja knowledge related to Kaguya.
Sasuke would most likely be involved. Since he has a strong bond with Naruto, he would sense something is wrong and try to free Naruto from Kaguya's control. Sakura might also be part of it as she cares about Naruto and would be determined to save him. Kakashi could play a role too, using his wisdom and experience to figure out a way to break Kaguya's hold on Naruto.
Hagoromo Otsutsuki is often a popular character. Since he is related to the Kaguya Clan, fanfiction writers like to explore his backstory and his connection to the clan's powers in more depth. His role in the creation of ninjutsu also makes him a fascinating character to include in Kaguya Clan - related fanfictions.
Naruto is a must. Kaguya, as a key part of the Naruto lore, would be in it. In terms of the harem, Ino could be one of the characters. She has always had an interest in Naruto to some extent. Tenten could also be part of it, perhaps drawn to Naruto's new Saiyan - like strength. And from the Saiyan side, Raditz could be introduced, creating some conflict as he initially tries to take advantage of Naruto's Saiyan powers but later might become an ally. There could also be some original characters created just for this fanfic.
Sasuke can also be involved in some cases. Since he and Naruto are often together in many situations, Sasuke's presence might add some complexity to the relationship between Kaguya and Naruto. For example, Sasuke might be suspicious of Kaguya's intentions towards Naruto at first.
Yes, there are likely some fanfictions that explore a special connection between Naruto and Kaguya. In some fanfictions, authors might create scenarios where Naruto, due to his unique abilities and kind - hearted nature, manages to reach out to Kaguya in a way others couldn't. This could involve him understanding her past and trying to bring some form of redemption or peace to her.
The Otsutsuki clan members other than Kaguya could be antagonists. Since Kaguya is training Naruto, they might see Naruto as a threat to their plans.
Obviously Naruto and Kaguya are the most important. Naruto as the one to free Kaguya, and Kaguya as the one being freed.
Obviously Naruto would be one of the main characters since it's about him being the heir of Kaguya. Sasuke could also be a main character as Naruto's rival and friend. Sakura might be involved too, as she is part of Naruto's team. And there could be other characters like Kakashi, who would play a role in guiding Naruto as he discovers his new identity. There might also be some new characters introduced that are related to Kaguya's past, who either help or hinder Naruto on his journey.
It's also possible that some non - human characters could be part of the harem. For example, some of the tailed beasts in human form (if that's possible in the context of the fanfiction). They would bring their own set of powers and a very different kind of relationship with Naruto compared to the human characters in the harem.
In the fanfiction, Naruto could enter Kaguya's mind. He might discover her past traumas that led her to become so power - hungry and vengeful. By understanding her pain, Naruto could offer her forgiveness and a new path. He could show her a vision of a future where she is part of a united world, not feared or hated, but respected and loved if she changes her ways. This could be a powerful incentive for Kaguya to change.