In this crossover fanfic, the power of LOVE and EXP from Undertale could have an interesting effect on the FNAF animatronics. Maybe the animatronics gain some sort of sentience upgrade or change in behavior based on these concepts. And the FNAF setting could add a new level of horror to the generally more light - hearted (in some ways) Undertale world. For instance, the animatronics could start to infect the Underground with their own brand of fear, causing chaos among the monsters.
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.
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 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 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 the characters from Homestuck end up in the Underground of Undertale. They might have to deal with the various monsters there while also trying to find a way back home. Maybe John Egbert could team up with Frisk and they use their unique abilities to overcome challenges like puzzles and battles.
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 Pokemon are in the Underground world of Undertale. Maybe Pikachu could befriend Frisk and help them on their journey. The Pokemon could have unique interactions with the monsters there, like using their moves in battles against the monsters in a different way than in the Pokemon world.
One idea could be Sans from Undertale ending up in the Marvel universe. He could use his shortcuts and bone attacks to fight alongside the Avengers against a common enemy like Thanos. His laid - back attitude would contrast with the more serious heroes in Marvel.
One idea could be that the Turtles end up in the Underground world of Undertale. They might have to navigate through the various regions, like Snowdin or Hotland, using their ninja skills. For example, they could use their stealth to avoid some of the more dangerous monsters. Another plot could involve the characters from Undertale being brought to the Turtles' New York. They would then have to deal with the Foot Clan and other threats in this new, strange environment.
Well, another plot could involve Sans and Papyrus coming to the RWBY world. Papyrus might be fascinated by the Grimm and try to befriend them, while Sans could use his powers to help the RWBY team in their battles against Salem. He could also share his knowledge of timelines with Ozpin.