Perhaps they face challenges from the magical world. Since Saber has a connection to the throne of heroes, there could be threats from other entities who don't want a married Saber. They would have to fight together to protect their relationship and their new life. Meanwhile, they are also growing closer and deeper in love as they face these difficulties side by side.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3). There are a lot of Fate series fanfictions there. Just type in relevant keywords like 'Future Shirou and Saber married' in the search bar, and it should bring up some stories that match your criteria.
Growth is also a prevalent theme. Both characters grow as individuals within their marriage. Shirou might become more confident and assertive, while Saber may learn to be more open and emotional. This is often shown through various events in their married life in the fanfiction, like dealing with financial issues or making big decisions together.
There's a fanfiction called 'Married Bliss: Shirou and Saber'. In this story, they get married and start a family. It focuses on how Saber adapts to a more peaceful life, away from the battles she was used to. Shirou is always there to support her, and their love for each other just grows stronger with each passing day.
Well, 'Eternal Bond' is a really nice fanfic on this topic. It starts with their wedding and then goes on to show how they support each other in their respective goals. They have deep conversations and grow even closer as a married couple. There are also some really cute fanarts associated with it that enhance the reading experience.
Many fanfictions explore their relationship by showing their daily life. For example, they might show how Saber and Shirou communicate in the morning, like Saber making tea while Shirou reads the news. It gives a sense of normalcy to their relationship.
In fanfiction, Shirou and Saber's love is often depicted as a complex and beautiful relationship. It's a love that transcends their different natures. Shirou, with his strong sense of justice, is drawn to Saber's noble and powerful presence. Their love story can be full of moments of understanding and mutual growth.
One common theme is love. Their relationship is often explored in different ways, from a slow - burn romance to an instant connection. Another theme is loyalty. They are both loyal characters, and in fanfiction, this loyalty is often tested and reaffirmed.
Character development is crucial too. If the fanfiction shows how Saber and Shirou have grown as individuals and as a couple, the kiss becomes more meaningful. It could be that through various trials, they have come to understand and love each other more deeply, and the kiss is a physical manifestation of that.