Another theme is family. Since they are part of a work family and also become like family to each other personally. Fanfics might show them celebrating holidays together as a new - found family or helping each other with family - related issues.
Love, whether it's platonic or something more, is also a common theme. Some fanfics gradually develop their relationship into a romantic one, showing how their feelings for each other grow over time. They might share a special moment that makes them realize their deeper feelings.
One common theme is friendship. Their strong bond as friends is often explored, like how they always have each other's backs. For example, in many fanfics, when one of them is in trouble, the other is right there to help.