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.
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 117. Early on, Fragrant and cloyingly sweet potatoes and corn were out of the pot, and Qi...; later, the chapter leans on The searing heat made him grimace in pain, but he didn't let go, and he....
Look to Chapter 38 for the clearest answer. It begins with Jiang Li had also observed the girl who had just entered., keeps building through Before arriving, she had briefly reviewed the profiles of the other three..., and lands on He Yun coldly turned his head and continued to maintain his aloof.
The best chapter pointer here is Chapter 59, "Kicked Out". It opens with After walking for about two or three hundred meters, the old man stopped... and closes on My phone, my cigarettes, my champagne, oh how I miss you......" He swore,....
The best chapter pointer here is Chapter 69, "Received Two Advertisements on the First Day". It opens with Since the phones issued by the program team couldn't log into Weibo, Wen... and closes on Wen Jiaojiao immediately hugged her arms proudly, specifically waiting to....
Chapter 57 is the strongest chapter-level answer because it starts from Lu Xingchao's numerous fans were stunned. and follows through on I'm saying, big bro, can you at least reply to me?.
Chapter 59 is the strongest chapter-level answer because it starts from After walking for about two or three hundred meters, the old man stopped... and follows through on My phone, my cigarettes, my champagne, oh how I miss you......" He swore,....
Chapter 99 is the strongest chapter-level answer because it starts from Seeing the man approach, one of the individuals with a long scar across... and follows through on Just as she was considering whether to practice a set of boxing, her phone....
A practical answer is Chapter 100, because it turns the idea into a concrete scene. The chapter opens with The barrage was about to die of laughter., adds pressure through Who would dare write a script like this!], and ends on Her eyes flickered slightly as she grabbed the hand..
Look to Chapter 95 for the clearest answer. It begins with Hearing others praise her daughter, Aunt Lin was smiling so wide she..., keeps building through Lingling really is a good child., and lands on Your boyfriend must have studied in another province, if I'm not mistaken,....
Look to Chapter 101 for the clearest answer. It begins with Jiang Li walked over to the slope where the two had fallen down., keeps building through Below the small hill was nothing other than a large patch of sludge., and lands on If you encounter any trouble in the future, you can come to me..