Finally, there's the world - building aspect. Combining the Naruto world with the world of Fire Emblem Fates requires careful thought. You need to consider how the two societies interact, what the political implications are, and how the different cultures blend. For example, how would the ninja villages in Naruto react to the presence of the kingdoms in Fire Emblem Fates? And vice versa? These are all things that need to be addressed when writing this type of fanfiction.
One challenge is getting the characterizations right. Naruto characters have very distinct personalities, and so do those from Fire Emblem Fates. You need to make sure that when they interact, it feels natural. For example, Naruto's boisterous and optimistic nature might clash or mesh in interesting ways with the more formal or stoic characters from Fire Emblem Fates.
The different power systems are also a hurdle. Ninjutsu in Naruto and the magic in Fire Emblem Fates operate on different rules. Deciding how they interact and balance each other in the fanfiction can be difficult. If you make one power system too overpowered compared to the other, it can make the story uninteresting. For instance, if Naruto's rasengan can easily defeat any Fire Emblem Fates enemy without any challenge, the story loses its tension.