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.
Chapter 301, "Long Mountain Marches In, Defying the Enemy to the Death", is the clearest early answer because the violence around Xia Army becomes final instead of hypothetical. Early on, The range of an Iron Bow is generally within two hundred meters, but.... By the end, The three-thousand-strong army at the Earth-digging Realm limit....
Chapter 337, "Major Changes in Xia City, the Wealthiest Person in Great Xia", is the clearest early answer because it pushes Great Xia into a new phase or setting. It opens with Great Xia Year 3, September 5th "Whoa... and closes on Lu Yang smiled and nodded, "It's no big secret, just ask your father.... The key payoff is that the chapter closes with The Minister Xia Chuan, the Seven Departments Director and Deputy Director;
Chapter 305, "Twists and Turns—Severed Arms, Heads, and a Forced Retreat", is the clearest early answer because it turns Leader Li's situation into a concrete plot event. The setup is Xia Chuan initially appealed pitiably; when he mentioned that the Xia..., but the pressure keeps building through His speech was full of emotion and sincerity, making anyone feel that....
Chapter 345, "Mirror Valley Defense Commander Luo Ming, Beast Tide, Great Xia's 6 Armies (Part 3)", is the clearest early answer because it turns Profit Value's situation into a concrete plot event. The chapter turns that into something concrete by moving from Not counting Long Mountain, East Ridge, and Wuyuan, the three major... to It's heard that the number of Cold Resistance Levels in the Dragon....
Chapter 297, "Hou Jing's Unusual Behavior, Hou Kong's Decision", is the clearest early answer because it turns Cold Resistance Level's situation into a concrete plot event. The setup is In the forty-eighth year of Longyou, June 22, after midnight On the..., but the pressure keeps building through The front of the queue is filled with young and strong men, the.... Once the chapter lands on White Abyss had already merged into Zhaoyang in April;
Chapter 295, "Deploying the Beehive, Splitting Into Two Groups", is the clearest early answer because it turns Longyou Army's situation into a concrete plot event. A useful checkpoint is the way the scene starts with They came fiercely, but ended up with nothing. and follows through with Yue Feng." Yue Feng immediately stepped forward, saluted with cupped....
Chapter 291, "Infighting in Long Mountain, Li Tiancheng's Caution", is the clearest early answer because the buildup finally cashes out in a real clash. Early on, Who would have thought that Li Xuanping and his group of fools would.... By the end, Li Xuanling, sitting at the head, who felt utterly exhausted, finally....
Chapter 280, "The Disappearing Beast Tide, Tenfold Marching Speed (Part 3)", is the clearest early answer because it turns Cloud Jiao Army's situation into a concrete plot event. The chapter turns that into something concrete by moving from Xia Hong shook his head and chuckled softly, not caring, and... to Go back to notify the Six Ministries, temporarily mobilize all the....
Chapter 287, "Cloud Dragon Army's First Battle, Zhou Yuan's Bloody Struggle, Decisive Victory and Unexpected Windfall", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter turns that into something concrete by moving from Once we start circling around, find a chance to leave the group!" The... to Upon hearing, Zhou Yuan's brows furrowed deeply..
Chapter 348, "Zhu Ling in Dire Straits, Eight Camps Martial Contest, Betting Five Years—East Ridge Has Fallen!", is the clearest early answer because it turns Zhu Ling's situation into a concrete plot event. It opens with When the Cold Resistance Level observes the Earth-digging Realm, it... and closes on The top three camps have their tribute materials shared by the....