A plot could revolve around the mystery of the baby's powers. Since Rias is a powerful devil, there could be speculations about what kind of abilities the baby will inherit. This could lead to various groups either trying to protect Rias and the unborn child or trying to use the baby for their own purposes. Rias would then have to fight to keep her baby safe while also preparing for motherhood. The story could also show how her friends and allies in the Highschool DxD world support her during this time.
Well, a possible plot could be Rias having to balance her pregnancy with her studies at school. She's not just a devil but also a student. There could be comical moments like when she has sudden cravings during class or struggles to fit into her school uniform as her body changes. Also, the relationship between Rias and Issei could deepen in new ways. They might have to face challenges together, like dealing with over - protective family members from both the human and devil sides.