Masou Gakuen HxH aka Hybrid x Heart Magias Academy Ataraxia Challenge

Spoiler alert for those who haven't read the novels. There are 14 main novels. The manga only covers the first three and the anime only covers the first four. In the final story arc, the portals between Lemuria and Batlantis were about to close forever. Kizuna Hida slept with Grabel and Zelshione before they returned to their respective worlds. Grabel became ruler of Izgard and married her girlfriend Aldea before giving birth to a daughter, whom she named Kizuna, while Empress Grace Sinclavia became the child's honorary aunt. Meanwhile, when Zelshione found out she was pregnant, she went AWOL from her post as Captain of the Batlantis Guard and fled to the countryside, where she gave birth to a son, the first male born in Batlantis in centuries, whom she also named Kizuna. She was eventually found and arrested, then she and her baby were brought to Grace to stand trial for treason. Grace became enamored with the son of the great warrior Kizuna, so she "punished" Zelshione by dismissing her from her post and sentencing her to house arrest in the palace until she raises her son into a splendid person. Grace also planned to marry the boy when he comes of age, reasoning that a 15 year age difference is not so bad. Eventually, the two worlds were able to briefly communicate, so Kizuna and his harem became aware that he is a father. Aine expressed a desire to reconnect the worlds so that they could meet their friends and the two children.

The challenge is this: Write a story about the two half-siblings as they grow up. They would likely inherit their father's chick magnet nature, so maybe write about how they both deal with a world of only women, especially the son and how he deals with his impending marriage to Grace, like if he resents her for keeping him and his mother prisoner. If they manage to reconnect the worlds, then show the half-siblings meeting their father for the first time, as well as any potential children he would have with the rest of his harem.

Either that, or write some kind of crossover where someone powerful takes Kizuna's place or some people stumble onto the war between Lemuria and Batlantis.