One challenge is getting the tones right. 'Made in Abyss' has a very distinct, often dark and mysterious tone. When crossing over with another work, maintaining that tone while integrating the other work's elements can be tricky.
There's also a crossover fanfic that pairs 'Made in Abyss' with 'Fullmetal Alchemist'. In this fic, the alchemists find themselves in the Abyss and have to deal with its strange rules and dangerous creatures. It plays on the contrast between the two universes' magic systems and the characters' moral compasses.
One challenge is maintaining the tone of 'Made in the Abyss'. It has a very distinct dark and mysterious feel, and when crossing over with another universe, it's hard to keep that essence intact. For example, if crossing over with a more light - hearted series, the writer has to find a balance so that the Abyss's mood isn't lost. Another challenge is character integration. Characters from different series have their own personalities and backstories, and making them interact in a believable way within the context of the Abyss can be tricky.
A crossover with 'Pokemon' would be really cool. Imagine Pokemon trainers descending into the Abyss, using their Pokemon to battle the dangerous creatures there. Some Pokemon might even have unique evolutions due to the strange environment of the Abyss. Also, a crossover with 'My Hero Academia' could work. Heroes and students could go into the Abyss as a new kind of training ground, facing the moral dilemma of using their powers in such a dangerous and uncharted place.
Character consistency is also a big issue. Each character in 'Made in Abyss' has their own motives, fears, and personalities. When bringing in characters from another story, you have to ensure that their actions and reactions are in line with their original selves while also making sense in the context of the Abyss. For instance, if you crossover with 'The Legend of Zelda' and bring in Link, his stoic and heroic nature should be maintained while also being affected by the Abyss's environment.
A simple fanfic could be about Reg and Nat sharing a quiet moment on the surface, away from the dangers of the Abyss. They could be talking about their dreams and fears, getting to know each other on a deeper level.
One great fanfic could be 'Reg and Riko's Adventure Beyond'. It beautifully captures their relationship as they explore deeper into the Abyss. The author does an amazing job of showing Reg's protective nature towards Riko.
There are many possible fanfics for Reg and Nat in the Made in Abyss world. Some might focus on their adventures together exploring the Abyss. For example, a fanfic could be about them facing a new and dangerous creature while on a quest to a deeper layer.
There might be a fanfic that focuses on the backstory of one of the characters, like Riko. It could explore her early days in the orphanage, her training, and how she developed her strong will to descend into the Abyss. This could give readers a deeper understanding of her character.