One possible theme could be self - discovery. As the character is transported to a new world, they have to learn about themselves in a different context.
Since I haven't read it, it's hard to say for sure. But in general, isekai novels often have themes like self - discovery. The protagonist is usually thrown into a new world and has to figure out who they are in this new context.
Another theme could be adaptation. The protagonist has to adapt to the new environment, different cultures and rules in the isekai world. They start from zero, so they have to learn everything anew. This process of adapting can be a central theme that drives the story forward. There might also be a theme of forging new identities, as they leave their old self behind when starting this new isekai life.
The possible effects are quite diverse. If it's a healing - type ataerareta skill, it can save the lives of the character's companions, strengthening their bonds. It may also make the character more respected in the isekai society. On the other hand, if it's a destructive skill, it could cause destruction in the environment of the kasei world, which might have consequences like making some areas uninhabitable or angering certain factions in the novel's world.
The themes could include adventure and exploration. The idea of using magic to open up or control an isekai world implies that there will be a lot of unknowns to discover. There might also be a theme of friendship or camaraderie. In this new world, the protagonist may meet various characters and form bonds with them. Also, the struggle between good and evil could be a theme, as in many isekai stories, there are often forces that oppose the protagonist's actions in the new world.
Well, it might be about a character who enters an isekai world and tries to recruit kojiin for some purpose. Maybe they need kojiin's special skills or powers to achieve a certain goal in that strange new world.
Since it's an isekai, the theme of self - discovery could be present. The main character might be learning about themselves in this new world. Also, the 'dorei haaremu' part could introduce themes of social hierarchy and inequality. It could show how the character navigates a world with a different set of rules regarding slavery and relationships, and perhaps they strive to change or adapt to it.
One characteristic is the transportation to another world. This could be a medieval - like fantasy world, a post - apocalyptic world, or even a world based on different rules of magic or technology.