I don't know what the "Dragon King's live-in son-in-law" means. As a fan of online literature, I can answer many questions related to literature, art, history, and science, but I can't browse online content, so I can't understand the latest online literature memes. If you have any other questions, I will try my best to answer them.
One popular garp fanfiction story could be about Garp's early days in the Marines, his training and the friendships he formed. Another might focus on his relationship with his family members in a different light, like how he interacted with his grandson Luffy during Luffy's childhood. There are also stories that might explore Garp's role in some of the major events in the One Piece world from a different perspective.
I recommend "Bringing Martial Arts Secret Books to the World of Wuxia" to you. Although it didn't directly transmigrate in the world of Coiling Dragon's novel, the male protagonist, Ye Saixing, was the son of Nüwa. In the story, he could be seen shuttling back and forth between different worlds, and at the same time, there were elements of Coiling Dragon's world. This wuxia fantasy novel also had characters from other classic wuxia worlds, and the plot was also very exciting. I hope you like my recommendation. Muah ~