It would be a very difficult fit. Since the wizarding world has its own norms like monogamous relationships in most cases (like the marriages we see in the series). If one were to introduce concubines, it would have to be in an alternate universe where the social and moral values are completely different. Maybe in a fanfic where the wizarding world is modeled after a more ancient and permissive society, but this would be a huge departure from the original feel of the Potterverse.
To fit concubines into a Harry Potter fanfiction universe, one would need to re - imagine the social fabric of the wizarding world. For example, if the story was set in a time when wizarding society was more influenced by a culture that practiced concubinage. But this would also mean changing many other aspects like family structures, inheritance laws, and social hierarchies. It could also lead to a lot of conflict with the existing characters' values, as most of the main characters in Harry Potter uphold values like loyalty and equality in relationships.
Chapter 345, "Luo Ming, Mirror Valley Defense Commander, Beast Tide, Great Xia's 6 Armies", is the clearest early answer because it turns Redwood Ridge's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Great Xia, Year Seven, September Ninth, the latter half of the night... and follows through with In the first half of last year, Li Baihe, Xu Kang, and Shang Ping....
Chapter 411, "You Really Hold a Strong Grudge", is the clearest early answer because it turns Seeing Yu Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on The lead director, who understood the entire process, had an...
Chapter 281, "Zhao Lang, Are You Willing to Take a Gamble?", is the clearest early answer because it turns Gamble's situation into a concrete plot event.
Chapter 172, "Shang Shaojing is Here Too?!", is the clearest early answer because it turns Sister Li's situation into a concrete plot event. It opens with Not only Shu Yan, but the entire internet was shocked when they saw... and closes on Where's the chaotic scene I was anticipating?.
Chapter 243, "How Could It Be Her?!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event.
Chapter 222, "Oh, Miss Jiang Has Drawn It", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 50, "Girls Help Girls!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 86, "If you don't welcome me, why should I stay?", is the clearest early answer because it turns Xiao Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with After over an hour of fuss, it was finally time to eat.
Chapter 8, "Is He Shameless?", is the clearest early answer because it turns Jiang Lili's situation into a concrete plot event. Early on, The resentment that had barely calmed among the crowd surged back two.... By the end, Other people with a handsome and charming appearance often try to....
Chapter 220, "The program team actually invited such a high-level boss!", is the clearest early answer because Wen Jiaojiao finally turns buildup into a real advancement. It opens with I know everyone has many questions, and I'm here to answer them one...