Perhaps the foxy is a misjudged character. The security guard starts to keep a journal about their encounters with the foxy. As he writes more, he begins to understand the foxy's motives and actions better. They end up on a journey together, where the security guard has to defend the foxy from other overzealous security guards who just want to get rid of it. In the process, they build a deep bond based on mutual understanding and respect.
In the case of the foxy, you can show its development through its body language. At first, it might be all hunched and ready to attack, but as it bonds with the security guard, it stands taller and more relaxed. The security guard's development can be shown through his internal thoughts. He could be constantly thinking about how wrong his initial assumptions about the foxy were. He starts to advocate for the foxy among his colleagues, which is a big step in his character growth.
One popular foxy x night guard fanfiction might be 'The Unlikely Bond'. In it, the night guard discovers Foxy has a softer side. Another could be 'Foxy's Redemption', where Foxy and the night guard form an alliance to face the other animatronics. And there's also 'Midnight Encounters' which focuses on their strange and tense interactions.
A possible plot is that in the fanfiction world, there's a glitch that causes Springtrap and Foxy to swap abilities for a short time. This leads to humorous situations as they try to adapt to their new powers. Also, a plot where Springtrap discovers Foxy has a hidden talent, like painting or singing, and this changes his perception of Foxy completely.
One idea could be that Funtime Foxy and Baby start off as rivals in the animatronic world but then find common ground when they are faced with a malfunction that threatens all of them. They then team up to fix the problem and in the process, develop a unique friendship.
One idea could be that Foxy and Toy Bonnie are from different 'groups' in the animatronic world. They initially have misunderstandings but gradually discover they have a shared love for music. They start to secretly meet and create their own unique tunes together, which then leads to a newfound friendship and they have to face the reactions of the other animatronics.
A plot could involve the human getting lost in the world of Foxy and Bonnie. Foxy and Bonnie then have to protect the human from other more dangerous characters. As they do this, they start to develop a deep bond with the human. Maybe the human has a special power that they don't know about until they are in danger, and this power ends up saving Foxy and Bonnie in a crucial moment.
One idea could be that Foxy and Freddy have to team up to solve a mystery in the pizzeria. Maybe there are strange noises at night and they suspect a new animatronic is causing trouble.
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.
One idea could be a story where Freddy and Foxy are in a lemon orchard and they discover a special lemon that has magical powers. They then go on an adventure to protect this lemon from an evil force. Another could be that they compete in a lemon - making contest and through the process, they develop a deeper understanding and friendship.
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.