One interesting idea could be exploring the darker sides of the characters. For example, showing how Naruto might struggle with more internal demons. Another could be re - imagining the story in a post - apocalyptic setting within the Naruto universe. Maybe Konoha has been destroyed and Naruto has to lead the survivors in a harsher world.
Naruto himself is often a popular character in Naruto Grim Tales fanfiction. His journey can be re - written in much darker and more complex ways. Sasuke is also popular as his relationship with Naruto can be explored from a different, more somber perspective. Sakura too can be a key figure as she might have to face harsher challenges and grow in unexpected ways.
One Grim Tale is 'The Monkey's Paw'. In this story, a family is given a mummified monkey's paw that has the power to grant three wishes. But every wish comes with a horrible, unforeseen consequence, leading to a series of tragic events.
The mystery in them. For example, in many of these stories, there are elements that are not fully explained at first. This makes the reader keep on reading to find out more. It's like a puzzle that you want to solve.
One possible plot could be that in the world of Grim Tales Junior, Minnie finds herself accidentally transported there. She teams up with the main characters of Grim Tales Junior to solve a mystery that threatens both her world and theirs. Maybe there's a dark force stealing colors, and Minnie, with her positive and cheerful nature, helps to bring back the joy and color to the Grim Tales Junior world.
Perhaps a powerful witch who wants to merge the Grim Tales Junior world with a nightmarish version of the Disney world. She sees Minnie as a threat because Minnie represents the light and hope of the Disney world. So, she devises various traps and illusions to stop Minnie and the Grim Tales Junior characters from stopping her evil scheme.
If you cross it with 'Harry Potter', you could include Harry himself. His magical abilities and his experience with the dark arts could be an interesting addition. He could interact with the Grim Tales characters, perhaps helping them with a magical problem they have in their own world.
One idea could be to cross it with a popular fantasy series like 'Lord of the Rings'. The dark and spooky elements of Grim Tales could blend well with Middle - earth's magical creatures and landscapes. For example, the main character from Grim Tales could find themselves in Middle - earth and have to deal with orcs and other threats while also trying to find a way back home.