A more complex fanfiction could involve a threat that affects both universes. Ben and the characters from Highschool DxD have to team up. Rias could use her devil powers in combination with Ben's alien abilities to defeat an enemy that is trying to merge the two worlds in a bad way, causing chaos everywhere.
One idea could be that Ben Tennyson somehow ends up in the world of Highschool DxD. Maybe his Omnitrix gets a strange power-up that transports him there. He could use his alien forms to interact with the characters in DxD, like using Four Arms in a fight against some of the devils or fallen angels.
Another idea is a crossover where the characters from Highschool DxD discover the Omnitrix. Issei might be really excited about it and try to figure out how to use it. This could lead to some really funny situations as he tries to transform into different aliens but doesn't quite understand how it works at first.
If you want the clearest chapter reference, go to Chapter 78. Early on, He Yun, too, tidied up and walked over to rest in the shade of a tree.; later, the chapter leans on After more than an hour of basking in the sun, his cheeks were....
Chapter 81 is the strongest chapter-level answer because it starts from Qi Tianyu, who chose curry beef rice, received a piece of curry and a... and follows through on The summer on the Southwest Plateau is no joke; the sun at three o'clock....
Look to Chapter 87 for the clearest answer. It begins with Luckily, at that time, Jiang Li had already taken her pot of stinky..., keeps building through But all this still didn't escape the high-definition camera., and lands on I'm driven crazy by him every day.] [+1 from above!.
Look to Chapter 119 for the clearest answer. It begins with What is Princess Wen talking about?, keeps building through I can't understand a word?] [Is this some new meme?] [!!!, and lands on For some reason, he always felt there was a mysteriously calming power....
Look to Chapter 86 for the clearest answer. It begins with After over an hour of fuss, it was finally time to eat., keeps building through Qi Tianyu, starving, crazily scooped up his curry rice into his mouth., and lands on But then her expression became resolute and arrogant..
Look to Chapter 108 for the clearest answer. It begins with In a glamorous Michelin three-star restaurant, Su Yinwan and Shang..., keeps building through The elegantly dressed waiters started streaming in, serving them various..., and lands on Hey, I've been waiting for you for so long, where have.
Look to Chapter 99 for the clearest answer. It begins with Seeing the man approach, one of the individuals with a long scar across..., keeps building through How is it, have those people outside left?" "They're gone." The man spat..., and lands on Just as she was considering whether to practice a.
Chapter 85 is the strongest chapter-level answer because it starts from Facing everyone's skepticism and curiosity, Jiang Li explained fluently:... and follows through on This fresh and vibrant flavor blended perfectly..
Look to Chapter 82 for the clearest answer. It begins with Jiang Li gave her a fleeting glance., keeps building through When did we ever make a private deal?" "Just now!" Wen Jiaojiao defended..., and lands on And so, aside from Wen Jiaojiao, the remaining five set off, each....
The best chapter pointer here is Chapter 96, "Can't Easily Forgive the Scumbag!". It opens with Jiang Li's words seemed absurd, yet there also seemed to be a trace of... and closes on Xu Ling couldn't contain her soft sobbing, "How could this happen?.