A human friend of the kids who is really into the supernatural could be an interesting addition. They might be the first to notice that there's something different about the babysitter and start investigating. This could lead to a unique friendship or even a love interest for the gender - swapped vampire babysitter.
A nosy neighbor character could be great. They might suspect something strange about the gender - swapped vampire babysitter and start poking around. It adds an element of mystery and danger as the babysitter has to be extra careful not to be discovered.
One plot idea could be that the gender - swapped vampire babysitter has to deal with different social expectations in the human world while still trying to hide their true nature. For example, they might struggle with how to interact with the kids in a more 'gender - appropriate' way according to human norms while also having to keep their vampire instincts in check.
For the vampires in the role of babysitters, you can make them seem out of place. For example, they might wear their usual gothic style but try to fit into a suburban babysitting job. They could be constantly confused by the normalcy of it all.
One idea could be that the vampires become the babysitters instead of the humans. They could struggle with the normal human tasks like making snacks or putting kids to bed in a very comical way.
It could be another vampire that she met in secret. Maybe he was passing through town and they had a brief encounter, and now she's left to deal with the pregnancy on her own.
Sasuke is always an interesting character to include. If Naruto is gender - swapped and in the Uchiha clan, Sasuke's reaction to this new Naruto would be very interesting. He might be more protective or more competitive. Itachi also plays a crucial role. His relationship with the gender - swapped Naruto could be full of mystery and depth. He could see her as a new hope for the Uchiha or be suspicious of her motives. Kakashi could also be included. His mentorship to a female Naruto in the Uchiha clan would be different. He might have different training methods or advice for her compared to the male Naruto.
Definitely the babysitter. Writers can explore her backstory, her struggles with being a vampire and trying to fit into the human world while babysitting. The main kid she babysits is also a popular choice. He's the one who has the most interactions with the babysitter and often gets into crazy situations because of her vampire nature. Also, any of the villainous vampires from the show can be fun to write about, as they can add a lot of drama to the fanfiction.
Obviously, Erica's sister is a main character. And we can expect characters from the original 'My Babysitter's a Vampire' series to be involved as well, like the babysitter who is often in the middle of the vampire - related chaos. There may also be other vampires and humans who interact with Erica's sister throughout the story.
Definitely a magical gnome. Gnomes are often associated with magic and mischief, and they could interact with the little vampire in fun ways. And an angel character would be great too. The angel could represent the good and the vampire could be on the more 'grey' side of morality, leading to some great character development moments.
One great Vampire Benny fanfiction is 'Benny's Hidden Powers'. It really delves into Benny's character and his unique vampire abilities. The story has some exciting plot twists that keep you hooked from start to finish.
Obviously, the vampire babysitter herself is a great character to focus on. You can explore her internal struggles, like how she balances her vampire instincts with her job of taking care of the kids.