A really interesting plot could be that Gregory gets lost in the human world during the day. He has to find his way back home while avoiding being discovered by too many humans. He could meet a kind old lady who helps him without really knowing what he is, and through this encounter, he learns more about human kindness.
One idea could be exploring the vampire's family history more in - depth. Maybe there are ancient secrets or rivalries within the vampire clan that the main characters could uncover.
Perhaps Rudolph discovers a hidden power within himself that helps him protect his vampire family from a new threat in their town. This could involve a series of challenges where he has to learn to control this power.
Perhaps the fanfiction could explore a friendship between the little vampire and a human who discovers his secret. They could go on adventures together, with the human helping the vampire understand human emotions better, and the vampire using his powers to protect the human from danger.
One idea could be the little vampire befriending a human who is also a slasher movie fan. They bond over their differences and similarities, like the vampire's nocturnal nature and the human's love for horror. The vampire could show the human the 'vampire version' of horror, which is their everyday life.
A great plot could be that Glamrock Freddy and Gregory find an old animatronic from a previous iteration of the Pizzaplex that holds the key to an ancient mystery. They have to work together to figure out what the mystery is and how to use the key, all while avoiding the other animatronics who are acting strangely.
One idea could be that Vanessa and Gregory are on a mission to uncover the secrets of the old parts of the building they're in. They face various challenges together and their bond grows as they rely on each other's skills. For example, Gregory's quick thinking and Vanessa's knowledge of the place help them avoid the security mechanisms.
One idea could be that Gregory is lost in the spooky FNAF world and Vanessa, who initially seems menacing, actually turns out to be his protector. She could use her knowledge of the place to keep him safe from the animatronics.
One great 'Little Vampire Gregory' fanfiction is 'Gregory's New Adventure'. It takes Gregory into a modern city and shows how he adapts to new technology while still being a vampire. It's really interesting to see how the author blends the traditional vampire traits with modern settings.
Maybe the OC has a special power or ability that Rudolph needs. For example, the OC can sense when danger is near. They team up to protect Rudolph's vampire family from a group of hunters who have come to town. As they work together, a deeper bond forms between them.
One plot idea could be that Vanessa discovers Gregory is not just an ordinary child in the mall. She realizes he has some special connection to the animatronics or the secrets of the place. Maybe Gregory is the key to unlocking a mystery that has been hidden for years. For example, he could be related to the original creator of the animatronics in some way. Another idea is that Vanessa tries to protect Gregory from the more malicious animatronics while also trying to figure out his true identity. They could form an unlikely alliance and start to trust each other as they face the various threats in the mall together.