One cute moment could be when Gajeel, who is usually tough, tries to write a poem for Levy in English class but messes it up hilariously. Levy finds it endearing though. Another moment might be when they are paired up for a science project. Gajeel is all clumsy with the equipment, but Levy patiently teaches him how to use it, and they end up having a great time together.
One cute moment could be when Serena bakes a special Poké-pastry for Ash. In fanfiction, she might put in extra effort to make it look like a Pikachu. Ash's reaction of pure delight would be really sweet.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
One cute moment could be when Naruto finally notices Hinata's shy glances during a school assembly. He might give her a big, goofy smile that makes her blush even more. It could be the start of their connection, a simple yet sweet moment that makes Hinata's heart flutter.
Asta could be brought into the High School DxD fanfic as a result of a multiverse - related event. In this case, different universes are colliding or being connected in some way. His presence would disrupt the power balance as his anti - magic is something that no one in the High School DxD world has really encountered before. He could end up training with the devils to learn more about their magic while also teaching them about the concept of anti - magic.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.