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 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.
A common theme is power and its corrupting influence. Orochimaru, in his pursuit of power, abuses Sasuke who is also seeking power. Another theme is the loss of innocence. Sasuke, who starts off as a somewhat naive boy, gradually loses his innocence due to the abuse.
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.
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.
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.
Another plot could involve a sort of magical exchange. Orochimaru, being a master of some dark and strange jutsu in his own world, might offer to teach some of his knowledge to certain dark wizards in the Harry Potter world in exchange for something valuable. Harry Potter and his friends would have to prevent this knowledge transfer as it could tip the balance of power towards the dark side in their world. They might need to seek help from some of the more powerful and wise wizards like Dumbledore to outwit Orochimaru.
Perhaps the fanfiction could focus on the relationship between Orochimaru and Baby Mitsuki as a father - son type of bond. Orochimaru, having created Mitsuki, might have some paternal feelings. The story could show how Orochimaru tries to raise Mitsuki in his own way, teaching him about the ninja world, and how Mitsuki grows up with a unique perspective due to his creator. For example, Mitsuki might question Orochimaru's more devious actions while still having loyalty towards him.
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.
One possible plotline could be a more metaphorical 'pregnancy' in the sense of Orochimaru creating or nurturing a new and powerful jutsu or experiment that is like a 'baby' to him. It could show his obsessive nature in a different light, as he devotes all his time and resources to this new creation.
A possible plotline could be that they are both on a mission to rescue their kidnapped comrades. They have to work together, using their unique ninja skills. Female Sasuke with her lightning - fast attacks and Sakura with her super - strong punches.