The new spouse could be a character with his own backstory. He might be someone who was drawn to her strength in dealing with being a teen mom. Develop his character by showing his insecurities too. For example, he could be insecure about whether he can be a good father to her child.
Another idea is that her past as a teen mom could come back to haunt her in unexpected ways. Perhaps an old acquaintance from her teen mom days resurfaces and causes trouble in her marriage, like spreading rumors or trying to create jealousy between her and her spouse.
Traveling can be another way for the characters in the 'married at 23 fanfiction' to develop personally. They could go on a trip together or separately. Through traveling, they will encounter different cultures, meet new people, and be exposed to new ideas. This will expand their minds and help them figure out who they are and what they want in life. It might also have an impact on their relationship, either strengthening it or making them realize what needs to change.
Natsu would probably become more responsible. His wild and carefree nature might be tempered a bit by his marriage to Irene. He'd still be the same fun - loving guy but would also think about Irene and their relationship. Irene, on the other hand, might soften up. She was once a very powerful and somewhat cold character, but being married to Natsu could bring out her more caring and gentle side.
Starfire might develop by becoming more in tune with Earth's culture. Since she's married to Nightwing, she has a closer connection to Earth now. She could start to understand human emotions on a deeper level and become more empathetic towards the people of Earth. For example, she might learn how to handle complex human relationships better, like dealing with Nightwing's friends and family.
To develop Ron's character, you could show his insecurities about becoming a father. Maybe he has a fear of not being able to provide for the family, so he starts taking on extra jobs or learning new skills.
Ichigo would develop a more patient side. Halibel's different way of thinking and her more reserved nature would force Ichigo to slow down and really think about his actions. He would also become more protective, not just of Halibel but also of the new relationship they are building. Halibel would start to open up more. She was always a bit of an enigma in the original story. But in this fanfiction, being married to Ichigo would make her more expressive. She would start to share her thoughts and feelings more freely, and her character would become more complex as she tries to balance her old self with the new self that is influenced by Ichigo and his world.
In these fanfictions, Harry often develops a different side of his personality. He becomes more attuned to the social and family aspects of life. Daphne, on the other hand, may grow in her magical abilities as she shares knowledge and experiences with Harry. Their relationship helps them both to become more well - rounded characters. For instance, Harry might learn about the more refined aspects of wizarding society from Daphne, and Daphne might learn about courage and self - sacrifice from Harry.
Sheldon might become more empathetic. Being married to Penny, he is exposed to her emotions more often. For instance, when Penny has a bad day at work, Sheldon has to learn to comfort her, which is something he wasn't very good at before. So he starts to develop his emotional intelligence.
In a married life story, Kiba would develop a deeper sense of loyalty. His loyalty to Hinata would be on a whole new level compared to his loyalty to his friends or the village. He would be more protective and would do anything to keep her safe. Hinata would grow in her communication skills. Since Kiba is more outspoken, she would learn to express her feelings and thoughts more freely, which would also help her in her ninja work as she could better communicate with her teammates.
In married life, Eren could develop a more domestic side. She might start cooking for Levi, which is something new for her. Levi would have to adjust to having someone so close. He would learn to share his thoughts and feelings. He might also become more protective of Eren, especially if there are threats to their relationship or their lives. Their marriage would be a journey of mutual growth and understanding.