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.
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.
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.
If you want the clearest chapter reference, go to Chapter 84. Early on, When the villagers tasted his curry beef one by one, their faces revealed...; later, the chapter leans on In the end, he could only secure a score of 5..
If you want the clearest chapter reference, go to Chapter 50. Early on, [666, if good scores determine rankings, is Flash Change's production team...; later, the chapter leans on This is too tragic for this group of people.]" "[I'm dying of laughter,....
Look to Chapter 51 for the clearest answer. It begins with Oh, okay." After uttering two syllables indifferently, Jiang Li bent down..., keeps building through By the time she reacted, Jiang Li had already disappeared at the end of..., and lands on If I opened a bar here, wouldn't I become a.
The best chapter pointer here is Chapter 40, "The National Museum Speaks!". It opens with [Jiang Li claims she copied this herself? and closes on The handsome guys on this show are so difficult to deal with..
If you want the clearest chapter reference, go to Chapter 76. Early on, Thinking of this, the aunt repeatedly praised, "Girl, hearing your words...; later, the chapter leans on I'll go back and tell this to my child; he will definitely be motivated to....
A practical answer is Chapter 38, because it turns the idea into a concrete scene. The chapter opens with Jiang Li had also observed the girl who had just entered., adds pressure through When did Jiang Li become so rational and composed?!, and ends on He Yun coldly turned his head and continued to.
A practical answer is Chapter 51, because it turns the idea into a concrete scene. The chapter opens with Oh, okay." After uttering two syllables indifferently, Jiang Li bent down..., adds pressure through With this in mind, she looked up and scrutinized the worried faces in..., and ends on If I.
The best chapter pointer here is Chapter 87, "Temporary Replacement". It opens with Luckily, at that time, Jiang Li had already taken her pot of stinky... and closes on I'm driven crazy by him every day.] [+1 from above!.
If you want the clearest chapter reference, go to Chapter 44. Early on, But the next move by the production team proved they hadn't forgotten the...; later, the chapter leans on Since the Southwest Plateau area was economically backward, the airport....
Look to Chapter 50 for the clearest answer. It begins with [666, if good scores determine rankings, is Flash Change's production team..., keeps building through This is too tragic for this group of people.]" "[I'm dying of laughter,..., and lands on What kind of heavenly being had such a sour.