Well, perhaps Naruto and Sasuke are forced to join Orochimaru. Maybe Orochimaru has some kind of leverage over Konoha that makes them comply. In this fanfiction, they could start to learn unique jutsus from Orochimaru but also have to constantly resist his more evil intentions. They might also meet some interesting characters within Orochimaru's lair.
In a fanfiction like this, Naruto and Sasuke joining Orochimaru could lead to a total shift in the Naruto universe. They could be sent on dangerous missions by Orochimaru that involve retrieving powerful artifacts. Along the way, they might face their inner demons as they struggle with the morality of working for such a shady character. Their relationships with their old friends in Konoha would also be severely strained, and the story could explore how they deal with that.
Perhaps the female Naruto joins Orochimaru because she wants to learn more about the secrets of the ninja world that she believes Orochimaru can unlock. She may be curious about the darker aspects of ninjutsu that are not taught in the normal ninja academies. But then she discovers that Orochimaru is using her for his own immoral experiments. She then teams up with some of Orochimaru's former followers who also want to break away and they plot to take him down from the inside.
In this fanfiction, Naruto might join Madara because of some sort of time - travel or alternate reality situation. He could find himself in a world where Madara's actions seem more justifiable. Naruto, being Naruto, would still strive for peace. So he might try to guide Madara towards a more peaceful solution while being part of his group. This could create a really unique dynamic where Naruto is constantly trying to nudge Madara in the right direction while also dealing with the other members of the group who may not be as open to change.
Another plotline could be that Orochimaru is forced to train Naruto. Maybe the Hidden Leaf Village has some sort of leverage over Orochimaru. During the training, Naruto discovers some dark secrets about Orochimaru's past and his experiments. This knowledge could help Naruto in his future battles not just against Orochimaru if he turns on him, but also against other enemies who might have similar ideas. Naruto might also gain some new physical abilities from Orochimaru's training, like enhanced speed or strength.
Another aspect could be the psychological impact on Sasuke. Orochimaru's abuse might make Sasuke question his own worth and his path. He could be constantly in a state of fear and self - doubt, and the fanfiction could explore how he overcomes these negative emotions and finally stands up against Orochimaru.
One possible plotline could be that Naruto, heartbroken from a failed relationship or the loss of a dear friend, throws himself into the dangerous and secretive world of ANBU to numb his pain. He might start off as a reckless agent, but gradually find purpose in protecting the village in a more covert way. This could lead to him making new, deeper friendships within the ANBU ranks.
A possible plotline could be Orochimaru's redemption. He realizes the error of his ways and joins Naruto in protecting the ninja world. He could use his vast knowledge of jutsu and forbidden arts to help Naruto against new threats.
One possible plotline could be that Naruto, having a fundamental difference in ideology with Sasuke, decides to take a stand. Maybe Sasuke is on a path that Naruto believes will destroy the village or their friends. So Naruto uses his newfound powers, perhaps a combination of Sage Mode and Nine - Tails chakra, to directly confront Sasuke. Naruto could try to break Sasuke's resolve through sheer force and the power of their friendship memories.
A plot could be that Orochimaru, seeking redemption in a way, starts to train Sakura in some unique and dark arts that he has mastered. Sakura, with her determination to become stronger, accepts this training but struggles with the moral implications. This could lead to an internal conflict within her as she tries to balance her new - found power with her values. Another possibility is that Orochimaru places a curse on Sakura for some reason, and she has to find a way to break it. In her quest to break the curse, she delves deeper into Orochimaru's lair and discovers more about his true motives and plans. Or perhaps Orochimaru kidnaps Sakura to use her as a bargaining chip against Sasuke. But Sakura uses her wits and ninja skills to not only escape but also turn the tables on Orochimaru, showing him that she is not to be underestimated.
Another plotline could be a more psychological exploration. Orochimaru has always been a bit of an enigma, and his love for Naruto could be a way to explore his own humanity. Maybe he has a change of heart because of Naruto and starts to question his past actions. This could lead to a journey of self - discovery for Orochimaru, while Naruto is left to figure out how to deal with this new side of his old enemy.
One possible plotline could be Naruto being forced to serve Orochimaru due to some sort of blackmail or threat against his friends in Konoha. Maybe Orochimaru has some information that could endanger Naruto's loved ones if he doesn't comply. Another aspect could be Naruto initially pretending to serve while actually trying to find a way to take Orochimaru down from the inside. This would add an element of mystery and deception to the story.