One idea could be Naruto is transported to the world of Little Witch Academia. He could attend the academy, but with his ninja skills, he'd be a very different kind of student. Maybe he'd try to use chakra instead of magic at first and create some interesting chaos.
The themes of friendship and believing in oneself from 'Little Witch Academia' could be further explored in the context of Kingdom Hearts. How the characters face the darkness and find the light within themselves while also dealing with the Heartless could make for a very engaging fanfic. The relationships between the 'Little Witch Academia' characters and the Kingdom Hearts regulars like Donald and Goofy could also be developed in unique ways.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often tags and filters that can help you narrow down to the specific crossovers between My Hero Academia and Little Witch Academia. Many authors post their creative works there, and you can find a wide variety of stories, from action - packed adventures to heart - warming friendships.
First, know the characters well from both series. Understand their personalities, powers, and backstories. This will help you create believable interactions in your fanfiction. For example, if you're writing about Izuku Midoriya from My Hero Academia meeting Akko from Little Witch Academia, you need to think about how their different outlooks on heroism or magic would play out.
Another storyline might involve a character from Kingdom Hearts getting lost in the 'Little Witch Academia' world. For instance, Riku could end up there and have to work with Akko to find a way back. Along the way, they could exchange knowledge about their different worlds and powers. This could lead to some interesting battles against local villains in the 'Little Witch Academia' world who are interested in Riku's power.
Diana would also make a great main character. If she were handcuffed, she would approach the situation with her usual elegance and intelligence. She might use her vast knowledge of magic to try and find a solution, and her interactions with other characters, especially those she normally wouldn't interact much with while handcuffed, could add a lot of depth to the story.