It's highly unlikely to be a common type of fanfic. Fanfiction often aims to build on the existing story, characters, and relationships in a way that is somewhat believable or at least in - keeping with the spirit of the original work. A concept like 'ben impregnates gwen' is so far - fetched and often against the grain of the normal portrayal of the characters that it would be an outlier rather than a common type of fan - created story.
I'm not sure specifically as it could vary greatly depending on the writer's imagination. But it might be a fan - created story exploring a relationship between Ben and Gwen in a rather extreme and often unorthodox way in the context of fanfiction.
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 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.
A 'Ben loves Gwen' fanfic could explore a different side of their relationship. It might show a more romantic aspect compared to the normal sibling - like dynamic in the show. Maybe it could be set in an alternate universe where they're not related and the feelings develop naturally.
Adventure might be another theme. Since they seem like characters who could be in exciting situations, the fics might focus on their adventures together. They could be exploring new places or facing challenges as a group.
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 the exploration of their relationship in different scenarios. For example, how they deal with challenges together.
One common theme is the exploration of their shared past. How they grew up together and how that might lead to deeper romantic feelings. Another theme could be the challenges they face as a couple, like dealing with their different personalities and powers. Also, there might be themes of secrecy, as they might not want others to know about their relationship at first.
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.