A plot could involve the night guard having strange dreams about mangle. In these dreams, mangle is trying to communicate something important. This could lead to the night guard investigating further into the mysteries of the animatronic world in the pizzeria, and as they do, they get closer to mangle on a more emotional level.
One idea could be a story where Foxy and Mangle team up to escape the haunted pizzeria. They face various challenges like dodging the other animatronics and solving puzzles within the building.
One plot idea could be that Mangle and Springtrap are both trapped in an abandoned part of the pizzeria. They start off as enemies but gradually come to understand each other's loneliness. Another idea is that they are both haunted by their pasts in the pizzeria and they find solace in each other's company while trying to escape the nightmares of the place. Maybe there could be a plot where they team up to protect the new animatronics from a common threat in the pizzeria.
One idea could be that the night guard has a special connection with one of the ponies. Maybe they share a common interest in magic, and the pony helps the night guard understand the magical elements in Equestria better.
One idea could be a human character getting lost in the Pride Lands and having to learn the ways of the animals to survive. They could form an unlikely bond with a member of the Lion Guard.
One idea could be a story where the Lion Guard has to face a new and mysterious threat in a lemon - filled jungle. Maybe there's a villain who is using the lemons to create some sort of chaos, like making traps out of lemon juice that can paralyze the animals. The Lion Guard then has to figure out how to overcome this unique threat using their individual skills.
One interesting plot idea could be that Foxy and Mangle team up to solve the mystery behind the strange happenings in the pizzeria. They use their unique animatronic abilities to uncover secrets.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you're likely to find some interesting Mangle x Night Guard stories there. Just use the search function and type in the relevant keywords.
Another plot could involve a human who has a deep knowledge of ancient Pride Lands lore. The Lion Guard discovers the human while on a quest to solve a mystery that is endangering their home. The human helps them decipher old symbols and tales, leading to the discovery of a powerful weapon or magic that can save the Pride Lands.
Perhaps a story where a secret society meets only in the dark of night. The main character stumbles upon their meeting and gets drawn into a world of secrets, power struggles, and ancient rites.
One idea could be exploring a different origin for the vampires in the 'Twilight' universe. Maybe they are not just a natural occurrence but were created by an ancient magic gone wrong. Another could be a story where a human character has the power to control the elements and how that changes their relationship with the vampires and werewolves.