One common theme could be the exploration of their relationship in different scenarios. For example, how they deal with challenges together.
One common theme could be adventure. Since Ben and Gwen are often involved in various adventures in the original series, fanfics might continue that trend with new challenges and foes. Another theme could be family bonds, as they are cousins and their relationship is an important part of the story.
Adventure is likely to be a theme. If these characters are from a story with an adventurous backdrop, fanfics may continue to explore new places and challenges for them. Also, character development is often a theme. Writers may take the opportunity to further develop the personalities of Ben, Gwen, Gwendolyn, and Lemon in ways that weren't fully explored in the original work.
One common theme could be friendship. Given the characters, they might have adventures together which strengthen their bond as friends. Another theme could be self - discovery. Each character might grow and learn more about themselves throughout the story.
One common theme is the development of relationships. How Ben and Gwen interact with the other characters in the harem, whether it's friendship turning into something more or dealing with jealousy. Another could be the exploration of different powers and abilities, as often these characters have unique skills.
One common theme is adventure. They often go on exciting adventures together, facing various challenges and enemies.
I don't recommend delving into the themes of lemon fanfiction. But for regular Ben and Gwen - related fanfiction, themes can be things like loyalty. They are always there for each other during their battles against aliens. Another theme could be their evolving relationship as they get older and face more complex situations in the series. Also, their individual character development, like Ben becoming more responsible and Gwen getting stronger with her powers, are often explored.
One common theme is growth. Both Ben and Gwen grow as individuals and in their relationship. Another theme is friendship, as their bond is often a central part of the stories. Adventure is also a big theme, since they are from the Ben 10 universe and usually go on exciting adventures together.
One common theme is the struggle with their superpowers while dealing with their feelings for each other. Since they both have powers in the Ben 10 universe, their relationship can be affected by how they use their powers and the responsibilities that come with them.
One common theme could be friendship. Ben and Sally Lemon might be depicted as having a strong friendship that develops into something more. Another could be adventure, where they go on exciting adventures together.
Another theme might be sacrifice. For example, Ben has to sacrifice something that is important to Gwen for the greater good, and that hurts her. It could be a relationship, an opportunity, or even a physical object that has sentimental value. Often in these stories, the hurt then leads to character growth for both Ben and Gwen as they work through the situation.