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.
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.
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.
Chapter 284, "You'll Never Get Rid of Me!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Bai Lu, also an actress, naturally saw through the insincere... and closes on She viciously slapped her phone onto the desk..
Chapter 416, "Such a Sensitive Nose, Born in the Year of the Dog?", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. Early on, Yu Li's words successfully choked Lady Nina.. By the end, Why did she have to end up with such a dumb teammate?!.
Chapter 383, "Doomsday Beehive", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event. It opens with Jiang Li's pale fingertips rubbed against the handle of the coffee...
Chapter 197, "Dear Brother", 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 Seeing that no one cared about what she said, Su Yinwan immediately...
Chapter 185, "We Are the Real Team", 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 He was so anxious that he immediately picked up the megaphone.
Chapter 526, "Has Jiang Li Turned Evil?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on At seven in the morning, the livestream began, and most of the...
Chapter 528, "Cursed", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, The netizens almost unanimously started to sympathize with Jiang.... By the end, The current balance is 89!] Su Yinwan: "...." She was tired..
Chapter 725, "Still Want to be the Second Young Master of the Jiang Family!", is the clearest early answer because it turns Jiang Minghai's situation into a concrete plot event.
Chapter 256, "Bonfire Feast", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. Early on, Turning around, Jiang Yan and Zhao Lang found their heads had been.... By the end, I was dazzled seeing Jiang Li trending today." Next door, Su Yinwan....
Chapter 223, "Pigpen Live Stream", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. The chapter gives readers a real answer by opening on So, for the first time, Jiang Yan, who had always styled himself as...