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 great fnaf nightmare crossover fanfiction is 'The Nightmare's Alliance'. It combines the terrifying elements of FNAF nightmares with other popular horror universes. The characters' interactions are really well - written, making it an exciting read.
You could have the characters from Undertale drawn in a more FNAF - like style. For example, make Sans' eyes glow like the animatronics' in FNAF. Or give Frisk a more robotic look while still maintaining their Undertale charm. The backgrounds in the fanfiction could also be a blend. The colorful and somewhat cartoony Underground of Undertale could be made darker and more spooky like the FNAF settings, with broken animatronic parts scattered around.
Definitely not a true story. FNAF is a work of fiction that takes place in a fictional world with its own set of rules and scenarios. It's all about creating a thrilling and immersive gaming and storytelling experience, not reflecting reality.
The FNAF backstory is full of mystery. There are rumors of hauntings, dark events at the pizzeria, and unanswered questions that keep fans guessing. It's a world filled with suspense and horror elements.