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.
The plot could revolve around their adventures together as a married couple. Maybe they discover a new magical threat and have to use their combined powers and skills, with Shirou's swordsmanship and Rin's magic, to protect their world. They also might start a family and raise a child with magical abilities, passing on their knowledge and values.
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.
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.
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.
One possible plot could be them adjusting to married life in a world filled with the challenges and magic from their respective backgrounds. Maybe they have to face enemies together who are against their union, and they rely on each other's unique skills to survive.
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.
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 plot idea could be that child Shirou discovers Saber's true identity as a king through a magical encounter. Maybe they go on a small adventure in the local forest, with Saber teaching Shirou about honor and courage in a child - friendly way.