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.
There could be a plotline where Orochimaru experiments on Naruto in a different way. Instead of trying to take over his body, he tries to enhance Naruto's powers. This leads to Naruto having to deal with new and unexpected abilities and the consequences that come with them.
One more plotline might be that Orochimaru and Naruto end up in an alternate dimension together. They have to rely on each other to find a way back home. Along the way, they learn more about each other's pasts and form an unlikely bond that changes their relationship in the original Naruto world.
Perhaps Naruto starts to have a connection to Orochimaru's other experiments or creations. He might find himself in a situation where he has to either help or fight these other 'products' of Orochimaru's mad science. Also, the fanfiction could explore how Naruto's friends react to what has happened to him. Do they trust him still? Or are they afraid of what Orochimaru might have turned him into?
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.
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 Naruto being captured by Orochimaru and forced to endure various genetic experiments. Maybe Orochimaru is trying to extract the power of the Nine - Tails from Naruto in a more invasive way. Another could be that Naruto's friends, like Sasuke and Sakura, attempt a daring rescue mission while Naruto is in Orochimaru's clutches and they face off against Orochimaru's minions.
Well, in such a fanfic, a plotline might involve Naruto's friends trying to rescue him. They could discover Orochimaru's lair where Naruto is being experimented on. Meanwhile, Naruto himself could be developing new and unexpected powers as a result of the experiments, which he then has to learn to control. And there could be a race against time for Naruto's friends to reach him before Orochimaru's experiments cause irreversible damage.
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.
Well, perhaps in the fanfic, Orochimaru tortures Naruto as a form of revenge against the Leaf Village. He might use his various snake - related jutsu to cause pain and harm to Naruto. For example, he could make snakes constrict Naruto, slowly squeezing the life out of him while also injecting him with some sort of venom that causes excruciating pain. And all this while, Naruto is trying to find a way to break free and defeat Orochimaru, relying on his inner strength and the memories of his friends and his training.
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.
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.
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.