There probably is. Firefly has a large and creative fan base. Many fans like to explore different relationships between the characters. While the term 'harem' might be a bit out of place in a non - adult context, there could be stories where Jayne has multiple friends or allies that form a sort of group around him. Check fanfiction forums too, where fans might discuss such stories.
One common theme could be the dynamics within the harem. How the different characters interact with Jayne and each other. Another might be adventure, as Firefly is set in a space - faring universe, so the harem and Jayne might be involved in various escapades.
Well, finding good Jayne x Harem Firefly fanfics can be a bit of a hunt. You might want to check out some Firefly fan communities on social media platforms. People there often share and recommend their favorite fanfics. Sometimes they even create lists of the best ones based on different pairings, so it could be a good place to start looking for Jayne x Harem stories.
A great Jayne - centered non - adult storyline could be Jayne accidentally getting involved in a time - travel adventure. He ends up in a different era of the 'Firefly' universe and has to figure out how to get back to his own time while also not changing the future too much. Another could be Jayne forming an unlikely friendship with an alien species that the crew initially distrusts. He has to convince the crew that this new species can be an ally. And there could be a story where Jayne's past as a criminal catches up with him in a way that forces him to face his demons and change for the better.
Jayne, who is usually rough around the edges, would have to soften up a bit. His relationship with River would transform from a somewhat distant one to a very close and interdependent one. River, with her pregnancy, would be more in tune with her emotions, and this could help Jayne open up more as well. They would start to communicate on a deeper level, sharing fears and hopes for their future and the future of their unborn child. For example, they might start having long talks at night about how they want to raise the baby, what values they want to instill, etc.
One possible plot idea could be that they end up pregnant due to some strange experiment gone wrong on the ship. Jayne, being his gruff self at first, is in total denial but slowly warms up to the idea as he sees River's vulnerability. River, on the other hand, has a more spiritual and accepting approach from the start, seeing it as a new chapter of life. They then have to deal with the reactions of the rest of the crew, some are supportive while others are worried about how it will affect their operations on the ship.
The Jayne Mansfield story is quite interesting. Jayne Mansfield was a famous American actress, singer, and Playboy Playmate. She was known for her blonde bombshell image in the 1950s and 1960s. She appeared in several movies and was a popular figure in Hollywood. However, her life was cut short in a tragic car accident. Her career had its highs and lows, with her being one of the sex symbols of her time, but also facing some criticism for her more provocative roles and public image.
It could potentially have characters like Zoe Washburne. She is a strong and loyal character in the 'Firefly' world, and it would make sense for her to be involved in the events of the unification war. There may also be new characters specific to this volume that we don't know about yet.