Another interesting concept is a time - travel crossover. Have a character from Highschool DxD travel back in time to an era in a different fictional universe. They could end up in the world of Game of Thrones and have to deal with the politics and power struggles there, while also using their DxD powers. It would be really cool to see Issei trying to survive in Westeros and maybe even form an alliance with Daenerys using his devil powers.
One idea could be to introduce characters from another popular anime into the Highschool DxD universe. For example, having Naruto characters in the DxD world. They could bring their unique jutsus and ninja skills, which would create interesting interactions with the devils, angels, and fallen angels in DxD. Maybe Naruto could use his shadow clone jutsu to outwit the devils in a battle of wits.
You could also do a crossover where the Highschool DxD characters end up in a post - apocalyptic world like that of The Walking Dead. The devils, angels, and fallen angels would have to work together to survive against the zombies. Their powers would be put to the test in a whole new way. For instance, Rias could use her magic to protect a group from a horde of zombies, while Akeno could use her lightning powers to electrocute large groups of the undead.
Chapter 126, "The Production Team Actually Played Dirty on This?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on Wen Jiaojiao initially wanted to cause trouble.
Chapter 107, "Are You Really Going to Participate in That Variety Show?", is the clearest early answer because it turns Teacher Chen's situation into a concrete plot event. The chapter gives readers a real answer by opening on On the plane, when He Yun had taken off his mask and greeted her...
Chapter 85, "They must have been bought off by Jiang Li!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Facing everyone's skepticism and curiosity, Jiang Li explained...
Chapter 121, "Jiang Li Gains 5 Points for His Meritorious Rescue!", is the clearest early answer because it turns Director Sun's situation into a concrete plot event. It opens with Seeing that Wen Jiaojiao's condition had improved significantly,...
Chapter 79, "Finally, I'm not the last one anymore!", is the clearest early answer because it turns Hua Country's situation into a concrete plot event. It opens with Jiang Li leisurely said, "I bought this weeding machine, and...
Chapter 116, "Group Freeloading!", is the clearest early answer because it turns Director Sun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with But before the audience in the live room had time to continue brewing...
Chapter 366, "The Safety Lock Got Flattened?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. Early on, You're finally out!" Xiao Wu saw Jiang Li and the others from a....
Chapter 30, "Lin Manru: You have to go even if you don't want to!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 170, "This Family Will Fall Apart Without Jiang Li", is the clearest early answer because it turns President Shang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Watching the two who had long remained silent, the PD shifted his...
Chapter 229, "He's Actually Brother Liu, the Watermelon Seller?!", is the clearest early answer because it turns Huo Zhenhua's situation into a concrete plot event.