The Adulterous Switcher - Chapter 37: Fire and Ice!!

The little handsome son-in-law passed through a world as waste material and the husband of a wealthy family. But because of his lacking aptitude, the wealthy family had driven him out. So he worked hard and found a more powerful family’s thighs to hold onto. Soon enough, he became the strongest son-in-law and husband in history.

In this world, it was impossible to practice martial arts. It was futile so he might as well take advantage of his wife’s thighs and maintain a comfortable and leisure lifestyle.

He’ll train his wife to become the strongest and best person in the world.

“Whoever dares to provoke me, let my wife kill you!”

- Description from Novelupdates

Rowen transmigrates into the world of One Piece, gaining the Strongest Trait System—capable of plundering the Traits of others!Spotting the golden—shimmering Trait on Koby, Rowen scoffs—You think a clown like you deserves this? Gimme that!From then on—Rowen plunders even more powerful Traits.Marine Headquarters: “He is a monster who devours and takes everything—the most dangerous ability user in history!Engaging him is forbidden without explicit authorization from the highest levels of the World Government!The World's Most Dangerous Man! 'Sky Sovereign' Rowen!DEAD OR ALIVE! Bounty: 3,988,900,000 Beri!”

[WARNING: MATURE CONTENT]

Another entry in what you can call the Proto Harem Series: the books written before the Harem Series that take place in the same world.

It’s about a swordswoman whose student is captured, so she goes to rescue her.