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.
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.
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.
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.
The themes could also include redemption. After kidnapping Marco, Tom might realize the error of his ways and try to make amends. And there could be a theme of friendship. Even though Tom has kidnapped Marco, the story could explore if their friendship can be salvaged.
One possible theme could be the combination of different power systems. Naruto has its chakra - based powers and Gundam has advanced technology. How these two interact in a fanfiction could be an interesting theme. Another theme might be the cultural clash. The ninja world of Naruto and the more futuristic, often military - focused Gundam universe could lead to some interesting cultural exchanges in the story.
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.
One possible theme could be self - discovery. If Draco is portrayed as a 'fat slob', he might go through a journey of realizing his own state and trying to change. Another theme could be social ostracism. People in the story might treat him differently because of his appearance.
Identity can be a theme. A character may question their identity as a Gundam pilot or a citizen of a certain faction when they fall in love with someone from the other side. And of course, the overarching theme of love itself. How it can transcend boundaries, whether they are political, ideological or related to the technology of the Gundam world.