A really interesting plot could be a time - travel one. Rukia somehow travels back in time to Naruto's childhood. She sees the lonely Naruto and decides to help him. She secretly trains him in some of her shinigami techniques, which Naruto then incorporates into his ninja style. This changes the course of Naruto's life and how he approaches his ninja training, and they form a deep connection along the way.
One plot idea could be a crossover adventure where Naruto and Rukia team up to face a powerful new enemy. Maybe they meet in a strange in - between world and have to combine their unique skills to survive. Another idea is a story where they exchange their powers for a short time and have to learn to adapt to each other's abilities in a comical yet challenging way.
One idea could be that Ichigo and Rukia find themselves in an alternate universe where their roles are reversed. Rukia could be the one with the Shinigami powers from the start in Karakura Town, and Ichigo is a normal human who gradually discovers his own latent spiritual abilities to help her. Another could be that they are sent on a special mission together to a realm neither has been to before, like a dimension full of lost souls. There, they have to rely on each other more than ever to find their way back and complete their task.
Another plot could be that there's a powerful enemy who tries to use Rukia and Ichigo's feelings against them. They have to fight not only the external threat but also their own growing desires for each other, which leads to some tense and steamy moments in the fanfiction.
One idea could be that their pregnancy is a result of some kind of magical or spiritual event in the Bleach universe. Maybe an ancient spell gone wrong or right, depending on how you look at it. For example, a spell that was meant to protect Soul Society actually ends up causing Rukia to become pregnant. Another plot could involve the two of them having to hide the pregnancy at first because it goes against some Soul Reaper rules or traditions. They then have to deal with the consequences of keeping it a secret while also preparing for the arrival of the baby.
One plot idea could be that Hollow Ichigo and Rukia are forced to team up against a new, powerful enemy that threatens both the Soul Society and the world of the Hollows. They start off distrusting each other but gradually learn to work together and understand each other's motives better.
One idea could be that Arrancar Ichigo has to protect Rukia from a new, powerful enemy in Hueco Mundo. Since he has Arrancar powers, he has a different fighting style and perspective, which could lead to some really intense battle scenes and a deeper understanding between them.
A plot could revolve around Sesshomaru being intrigued by Naruto's determination. Sesshomaru decides to train Naruto in some of his own combat techniques, which Naruto adapts to his own fighting style. In return, Naruto shows Sesshomaru the value of friendship and loyalty within a village. This could lead to Sesshomaru having a change of heart about his own solitary ways. Also, they could team up to face a common foe from Sesshomaru's world that has somehow entered the Naruto universe.
One idea could be that Naruto and Kashuina meet during a mission outside of Konoha. They start off as rivals, constantly trying to outdo each other in combat skills. But as they face various challenges together, they gradually develop a mutual respect and then deeper feelings. Another plot could be set in a post - war scenario where Naruto, now a hero, meets Kashuina who has a unique ability to heal the psychological scars of the war - torn ninjas. Their interaction then leads to a romantic relationship. And a third idea is that they are both students under a new sensei who pairs them up for special training. During this training, they discover things about each other that they never knew before and fall in love.
One idea could be Naruto being sent to the world of Death Note. He could use his ninja skills to try and outwit Light Yagami. Maybe he would initially be confused by the Death Note's power but then use his determination to stop Light from his killing spree. Another plot could involve Naruto meeting the Shinigami. He might learn about the balance between life and death in this new context and try to find a way to bring some of his ninja world's values to this strange new world.
Perhaps Naruto and Lala meet during a mission where Naruto is sent to a different dimension. Lala, being from an alien world, is intrigued by Naruto's determination and they form an alliance to face a common enemy in that dimension. Their different powers and abilities complement each other, and as they work together, they start to develop feelings for each other.