Well, one possibility could be that Naruto, with his Saiyan powers (assuming some sort of crossover mechanic), has to deal with Kaguya in a new way. Maybe he forms a harem that includes characters related to Kaguya's story. The plot could involve battles against new threats while also exploring the relationships within the harem.
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.
The Saiyan's power is mainly based on their physical strength, ability to transform, and use of ki. Naruto has his chakra, with the Nine - Tails' chakra being a major part. Kaguya has her own powerful chakra and unique abilities like the All - Killing Ash Bones. In a fanfic, the Saiyan's physical might could be a match for Kaguya's brute force attacks. Naruto's chakra - based jutsu could be used to complement the Saiyan's attacks, for example, using shadow clones to create diversions while the Saiyan attacks directly. Kaguya's reality - warping abilities would pose a huge challenge to both the Saiyan and Naruto, forcing them to come up with new strategies together.
One possible plot could be a crossover where a Saiyan somehow ends up in the Naruto universe during the time of Kaguya. Maybe the Saiyan's power is sensed by Kaguya and she sees it as a threat or an opportunity. The Saiyan, with their battle - hardened nature, could get involved in the Naruto characters' fight against Kaguya, adding a new dimension of power and strategy to the battle.
In fanfiction, the Naruto Reverse Harem Jutsu used on Kaguya could lead to some really interesting scenarios. Maybe it could be used as a comical moment to catch Kaguya off - guard. For example, she might be in the middle of some grand, menacing plan when suddenly the jutsu is cast and she's surrounded by a bunch of Naruto clones all vying for her attention in a rather silly way. This could potentially disrupt her concentration and give the protagonists an advantage in the fight.
Sakura could be in the harem. She has a long - standing relationship with Naruto in the original series, and in this fanfiction, his Saiyan powers might make her see him in a new light. Hinata is also a likely candidate. Her admiration for Naruto could turn into a deeper relationship given his Saiyan - like abilities.
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.
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.
Sakura could also be included. She is a strong and independent character. Her medical skills would be useful in the harem, and her relationship with Naruto, which has evolved over time, could add an interesting dynamic to the group.
One interesting plotline could be the exploration of different relationships within the harem. For example, how Naruto interacts with each member in a unique way. Maybe he has to resolve conflicts between the harem members who might be jealous of each other.