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.
Chapter 594, "Raising Soldiers for a Thousand Days, Using Them for a Moment", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. Early on, Thinking of this, Jiang Yan squeezed himself next to Zhao Lang while....
Chapter 749, "What if He Made a Fatal Mistake?", is the clearest early answer because it turns Fang Wenzhe's situation into a concrete plot event. The chapter gives readers a real answer by opening on I saw the trending topics." Jiang Li left Weibo and initiated a voice...
Chapter 508, "Isn't this the wild boar raised by Jiang Li?!", is the clearest early answer because it turns Isn's situation into a concrete plot event. It opens with A chill wind suddenly arose, and the team members of Team D hiding in...
Chapter 490, "Closing the Door to Let Pigs Loose", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 347, ""I Want to be the Eagle in the Wilderness", is the clearest early answer because it turns Eagle's situation into a concrete plot event. Early on, On the way there, Jiang Li had already learned quite a bit about the....
Chapter 510, "The Fake Young Master", is the clearest early answer because it turns Jiucheng's situation into a concrete plot event. The chapter gives readers a real answer by opening on Ever since that night's family banquet, Gu Qiang hadn't slept soundly...
Chapter 623, "Turned into a Little Crybaby", 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 Jiang Cheng woke up again, this time realizing he was being carried...
Chapter 379, "Is This the Way Huaxiu Treats Its Guests?", is the clearest early answer because it turns Huaxiu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Song Man chuckled twice, evidently displeased, and then positioned...
Chapter 379, "Is This the Way Huaxiu Treats Its Guests?", is the clearest early answer because it turns Liu Suqing's situation into a concrete plot event. It opens with Song Man chuckled twice, evidently displeased, and then positioned...
Chapter 656, "If he's the grandson of the richest man, then I am the richest man!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event.