The theme of mystery could be a big part of it. Why did Yun Fat kidnap Rain G? Is there some hidden agenda? The fanfiction could be centered around uncovering the mystery behind the kidnapping. Also, a theme of survival could be there. If Rain G is in the hands of Yun Fat, he or she would have to find a way to survive in that difficult situation, and this could be a major part of the story's development.
One possible theme could be rescue. If Rain G is kidnapped by Yun Fat, then the story might revolve around someone coming to rescue Rain G.
I'm sorry, I haven't come across such specific 'yun fat kidnaps rain g gundam fanfiction' stories, so I can't recommend any directly.
Perhaps Wong Yun Fat is being controlled by some evil force. This force compels him to kidnap Rain. Rain could be a key figure in some prophecy or something like that. So Wong Yun Fat's actions are not entirely his own will. He takes Rain to a secret location, and Rain has to use his wits to escape while also trying to figure out why Wong Yun Fat is doing this.
It could be Rain's best friend. This friend might be a very skilled fighter or detective - like character. He would start to search for Rain immediately after learning of the kidnapping. He would follow the clues left behind by Wong Yun Fat and his accomplices. And in a final showdown, he would rescue Rain from Wong Yun Fat's clutches.
Themes in this kind of fanfiction can be quite diverse. There could be a theme of mystery, like why Wong kidnapped Rain in the first place. Is there some hidden agenda or secret that Rain is a part of? Another possible theme is the relationship between the captor and the captive. It could start as a hostile relationship but develop into something more complex over time, such as an unexpected bond or understanding. Additionally, themes of survival and escape might be present as Rain tries to find a way out of the situation.
Perhaps in this fanfiction, Rain is from an opposing force to the G Gundam universe. She kidnaps the G Gundam fan as a way to gain information about the Gundams. The fan might be a super - knowledgeable person about G Gundam, and Rain wants to use that knowledge for her own purposes, like to develop a counter - measure against the Gundams.
Another theme might be that of discovery. Rain could be in the process of discovering the true nature of the dark gundam tentacles. This could lead to revelations about the Gundam universe itself, or about some hidden powers or secrets that are related to these tentacles. It might also be a journey of self - discovery for Rain as she faces this new and difficult situation.
Domon Kasshu could be the one to save the fan. He's the main character in G Gundam, and he's always ready to fight for justice. He would surely not stand by while a fan of his universe is kidnapped.
There is a fanfiction called 'Beyond the Gundam Tournament'. It takes place after the events of the G Gundam series. It explores what the characters do next and how they adapt to a world without the tournament. It has a lot of character development and some great emotional moments.
The rain could be related to the Dark Gundam itself. Maybe the presence of the Dark Gundam somehow affects the weather, and the rain is a result of its dark power. This could create an interesting plot point where characters have to deal with not only the Gundam but also the environmental changes it brings, like the constant rain.
Another idea is a crossover where the technology of G Gundam is introduced into the Naruto world. Maybe the Hidden Villages start to develop Gundam - like mechas using their own unique ninja techniques. This would completely change the dynamics of battles in the Naruto universe.