One challenge could be acceptance. Since Ichigo is from the human world and a Shinigami, and Nemu is an Arrancar, their union might not be easily accepted by either the Soul Society or the Arrancar community. Another challenge could be communication. They come from different backgrounds and cultures, so understanding each other's values and ways of thinking might be difficult at times.
In fanfiction, they might face the challenge of acceptance from their respective communities. Some in the Soul Society might think that Unohana is marrying beneath her status by choosing Ichigo, a human - turned - Soul - Reaper. In the human world, Ichigo's friends and family might have a hard time understanding his relationship with a Soul Reaper from a different dimension. Also, their different fighting styles and approaches to dealing with threats could cause friction in their relationship. For example, Ichigo is more impulsive while Unohana is more calculated. But through communication and compromise, they could overcome these challenges.
The medical aspect could be a huge challenge. In their world, it's not clear if there are proper medical facilities for a pregnant woman, especially one with spiritual powers like Rangiku. There could be complications during the pregnancy that are difficult to handle. Also, they might have to deal with the pressure of their families or superiors. If their families have different expectations for them, or if their superiors think the pregnancy will affect their duties, it could create a lot of stress for Ichigo and Rangiku.
One challenge could be cultural differences. Konoha and Suna have different traditions and ways of life. For example, the Hyuga clan has its own strict family rules which might conflict with Suna's more laid - back approach in some aspects. Another challenge is the distance between their villages. Maintaining a relationship while being so far apart for official duties can be tough. Also, their friends and families may have different expectations. Hinata's family might be worried about her moving to Suna, and Gaara's council in Suna may have concerns about a Konoha - native being so close to the Kazekage.
Another challenge could be Naruto's duties as a hero in his world. He might be called away often to deal with threats. This could leave Shion feeling lonely or neglected at times. She would have to be understanding of Naruto's responsibilities, and Naruto would have to find a way to balance his duties and his married life.
One challenge could be dealing with their different clan cultures. Naruto is from the Uzumaki clan and Hana from the Inuzuka clan. Their traditions regarding family, festivals, and even child - rearing might be different. Another challenge could be Naruto's dangerous missions. Hana would be constantly worried about his safety, and Naruto would have to balance his duties and his new family life.
One challenge could be time management. They are both important figures in the fight against evil in the Voltron universe. So, finding time for each other between battles and diplomatic meetings would be tough. For instance, Keith might be called away on a dangerous mission right in the middle of their anniversary celebration.
One challenge could be acceptance from their respective villages. The Leaf village might be hesitant to fully accept Haku, given his past association with Zabuza. Similarly, the Hidden Mist might have reservations about Naruto, a Leaf ninja. Another challenge could be jealousy from other characters who might have had feelings for either Naruto or Haku. For example, some of Naruto's female friends in the Leaf village might not be happy about his marriage to Haku.
One challenge could be dealing with different family expectations. Shizuru's family might have certain traditions they expect her to follow in the marriage, while Natsuki's family could have different ideas. For example, Shizuru's family might want a large, elaborate wedding ceremony, while Natsuki's family prefers a more simple and private affair.
The plot could involve Nemu saving Ichigo's life during a fierce battle. This act makes Ichigo see Nemu in a new light. They begin to spend more time together, sharing their past traumas and dreams. As they fall deeper in love, they decide to get married. However, they have to keep it a secret at first due to the political situation between Hueco Mundo and Soul Society. Eventually, their love story becomes an inspiration for others in both worlds.
They might face challenges like cultural differences. Ranma comes from a more traditional martial arts family, while Ryoga has his own unique background. There could also be issues with their living arrangements. Ryoga is often lost, so he might have trouble getting home on time, which could cause arguments. And of course, their friends' reactions could be a challenge. Some friends might not accept their relationship easily.