For an 'fnaf crossover fanfic', you could do a crossover with a post - apocalyptic world. The animatronics could be the last remnants of a failed amusement park in a world overrun by zombies or mutants. They might have to fight for survival just like the humans. Also, a crossover with a fairy - tale world would be interesting. Picture the animatronics in a world of castles and magic, where they are the dark force that the fairy - tale characters have to overcome.
One idea could be that the animatronics from FNAF end up in the Undertale universe. They might be mistaken for strange monsters at first. The Sans character could be particularly intrigued by their mechanical nature and try to figure out how they work.
One idea could be that Sonic and his friends accidentally end up in the FNAF universe. Sonic's speed could be a huge advantage against the animatronics. For example, he could quickly dash around the haunted pizzeria, confusing the animatronics and leading them into traps. Another idea is that the animatronics from FNAF somehow get transported to Sonic's world. The chaos that would ensue as they try to adapt to the bright, colorful and fast - paced Sonic world would be really interesting to write about. Maybe Sonic and Tails would have to figure out a way to send them back while also protecting their friends from the animatronics' unpredictable behavior.
One idea could be that the characters from Undertale find themselves in the FNAF pizzeria. Sans could use his shortcuts to avoid the animatronics' attacks. Another idea is that the FNAF animatronics end up in the Underground. Freddy might befriend Toriel, and she could try to 'fix' them. Maybe Papyrus would be terrified of the animatronics at first but then try to befriend them with his overly friendly nature.
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.
One idea could be that the characters from Undertale end up in the FNAF world. For example, Frisk could be trying to find a way to escape the haunted animatronics while also befriending some of them in a similar way they befriend monsters in Undertale. Sans could use his shortcuts to outwit the animatronics' movements. Another idea is to have the FNAF animatronics transported to the Underground. They might be seen as strange new 'monsters' and the Underground residents would have to figure out how to deal with them.
One idea could be that Foxy from FNAF crosses over into another universe where he teams up with a group of unlikely heroes. Maybe he uses his speed and stealth to help them in a battle against a common enemy.
One plot idea could be that the characters from MyStreet end up in the FNAF pizzeria. They initially think it's just a strange place but soon realize the danger of the animatronics. Another idea is that the FNAF animatronics somehow enter the MyStreet world and start causing chaos among the peaceful MyStreet community. For example, Freddy might try to take over the local diner in MyStreet, thinking it's his territory.
A possible plot could involve the souls in Undertale having some connection to the spirits that are rumored to be in the FNAF animatronics. The main character from Undertale, perhaps Frisk, could enter the FNAF world and try to free the trapped souls. In return, the animatronics could help Frisk in the Undertale world against some powerful foes.
Well, it could be that there is a glitch in the multiverse that causes a fusion of the two worlds. So we have characters like Frisk interacting with Freddy Fazbear. Frisk might try to show the animatronics a different way of living, more like the pacifist way in Undertale. And in the process, they all have to deal with the threats that exist in both worlds, like the purple guy from FNAF and the more malevolent monsters from Undertale.
One idea could be that the characters from Gravity Falls end up in the FNAF world. Dipper might be intrigued by the mystery of the haunted animatronics and try to solve it using his journal. Mabel could befriend the more friendly - looking animatronics, like Chica, and try to make them cute accessories. Another plot could involve the FNAF villains getting loose in Gravity Falls and the Pines family having to use the town's unique magic to stop them.