He might be shocked. In the fanfic, if he sees things like his friends' future achievements or the new threats that emerge, it would surely startle him. For example, if he discovers that one of his friends has a family of their own and has become a great leader in the village, he would be proud but also a bit surprised at how much has changed.
With his usual determination. Naruto has always been one to face difficulties head - on. So, even if he's shocked by what he sees in the fanfic's future, he'll likely try to find solutions immediately.
A common theme is the clash of two different universes. Naruto's ninja world with its magic and combat skills meets the horror - filled FNAF world. Another theme is the development of relationships within the harem. It could be about Naruto gradually getting closer to the various characters from both worlds.
There might be a fanfiction where the FNAF animatronics are brought into the Naruto world. The animatronics' strange and somewhat creepy nature would contrast greatly with the ninja world. The shinobi would be both intrigued and cautious of these strange mechanical creatures, and perhaps they would try to find a way to either use them or get rid of them.
The unique feature is the crossover aspect. Combining Naruto's ninja - filled world with the horror - filled FNAF world creates a very different and exciting setting. For example, Naruto's jutsu in the spooky FNAF locations make for some cool scenes.
One idea could be that the Naruto characters are trapped in the FNAF world. Naruto, with his determination, might try to find a way out by using his ninja skills. He could use his shadow clone jutsu to search different areas of the haunted pizzeria at once. Sasuke might be more interested in uncovering the secrets behind the animatronics, thinking it could give him some power or knowledge advantage.
Survival can also be a theme. Given the often - dangerous nature of the FNAF world, Naruto could be in a situation where he has to use his ninja skills to survive, with Fem Foxy by his side. And sometimes, there could be a theme of transformation or change, where either Naruto or Fem Foxy take on new abilities or characteristics due to their interaction with each other's worlds.
There are quite a few 'naruto fnaf harem fanfiction' out there. Some focus on how Naruto's skills from his own world translate into the strange and dangerous FNAF world while also building relationships with a group of characters in a harem - like setting. You can usually find them on fanfiction - specific websites by using the relevant tags to search.
One interpretation could be that it's a form of extreme fanfiction that combines two well - known series in a shock - factor way. Maybe the author was trying to create a new and disturbing narrative that stands out.