Well, a frequent theme in Homelander fanfic is his relationship with other characters. This could be his relationship with the other members of The Seven, or his interactions with normal people. It shows how his presence affects those around him. Another common theme is his origin story. Writers often like to create their own versions of how he became the Homelander we know. And the theme of his mental state is also common, like his megalomania and how it developed over time.
In the Boys comics, Homelander's story takes many twists and turns. He battles with his own inner demons and external enemies, and his fate is quite complex and unpredictable.