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.
Sonic would definitely be one of the main characters. His speed and heroic nature would make him a key figure in any crossover. Also, Freddy Fazbear from FNAF would be important as he is sort of the face of the FNAF franchise. Sonic could be the one trying to find a way out of the FNAF world or stop the animatronics, and Freddy could be the character that Sonic has to either defeat or befriend in order to achieve his goal.
Bonnie from FNAF could be a great crossover character with Foxy. They already have a connection in the FNAF universe, and in a crossover, they could have new adventures together, perhaps in a different time or place.
First, you need to be familiar with both the Sonic and Archie universes. Know their characters, settings, and storylines. Then, come up with an interesting concept for the crossover. It could be a simple adventure where the characters meet by chance, or a more complex plot involving a shared threat.
One possible plot could be that an ancient evil threatens both the Sonic X universe and the Sonic Boom universe simultaneously. Sonic from Sonic X and Sonic from Sonic Boom have to team up, along with their respective friends. They could start by accidentally crossing into each other's worlds through some sort of interdimensional rift. At first, they might be a bit confused and even a little competitive, but as they realize the scale of the threat, they combine their unique skills. Sonic X's Sonic is more used to high - tech environments and has different power - ups, while Sonic Boom's Sonic is more in tune with his natural island surroundings. Together, they could battle the evil and in the process, learn a lot from each other and their friends, ultimately saving both universes.
Eggman Nega could be a great main antagonist. He has the intelligence and the desire for power. He could find a way to open portals between the Sonic X and Sonic Boom universes to steal resources and technology from both. His plans would force Sonic X's Sonic and Sonic Boom's Sonic to team up against him. He could use his robotic creations from both universes, combining the advanced tech from Sonic X and the more rustic, yet effective, machines from Sonic Boom.
Good character development is key. Since it's a crossover, seeing how the FNAF nightmare characters interact and change in the new setting is important. For example, if they cross over with another horror franchise, how do they respond to new threats or allies? Another feature is a unique plot. It shouldn't just be a rehash of the original FNAF nightmare story but something that takes advantage of the crossover concept, like a new mystery or conflict.
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.
To write a good Steven Universe FNAF crossover fanfiction, you need to consider the tone. Since Steven Universe is more light - hearted in general and FNAF is horror - based, find a balance. You could start with the Steven Universe characters in their normal setting and then gradually introduce the FNAF elements. Let the characters react realistically to the new and scary situation. For instance, Amethyst might initially be curious about the animatronics but then get scared when they start acting strangely.
It could be really interesting. Maybe the gems from Steven Universe end up in the spooky world of FNAF. They could use their powers to fight off the animatronics. For example, Garnet's strength could be used to hold the animatronics back while Pearl uses her spear to disable them.