It could be a vampire hunter. He might move to Royal Woods and start suspecting that there is a vampire in the area, which of course is Lucy. He could be very determined and use all kinds of tricks to try and catch her.
A jealous vampire from a rival vampire family could also be the main antagonist. This vampire could be envious of Lucy's unique place in the Loud family and her ability to live a somewhat normal life among humans. So, he or she tries to disrupt her life and drive her away from her family and friends.
Perhaps an older and more powerful vampire who doesn't like Lucy's modern - day vampire ways. He might see her as a threat to the traditional vampire ways and try to stop her from doing things like going to school or being friends with humans.
Luna could also be a good match. Luna's laid - back and cool personality could contrast well with Lucy's more somber and mysterious vampire self. They could bond over their 'outsider' status in a way, with Luna being the rocker chick and Lucy being a vampire. Maybe they could form a band where Lucy uses her vampire - like charm to draw in the crowds.
Maybe Lucy could find a way to use her vampire abilities to help her siblings in a time of need. For example, when they are facing a mysterious threat in the neighborhood, her vampire powers like super strength or the ability to turn into a bat could come in handy.
The main antagonist could be a powerful and ancient fairy who has been banished long ago but now returns to seek revenge on the fairy world. And Lucy, being a new and important part of the fairy world, becomes a target. This antagonist might have unique and dark magic that makes it extremely difficult for Lucy to defeat. Also, the antagonist could have some sort of grudge against the fairy queen or king, and Lucy is caught in the middle of this power struggle.
A third plot idea is that a group of vampire hunters come to Royal Woods believing there is a vampire in the Loud House. Lucy has to use her wits and any vampire friends she might have made to outsmart the hunters and protect her family. This could involve some really exciting chase scenes and moments of clever deception.
A possible main antagonist could be Aizen from 'Bleach' who somehow gains the ability to control the powers from Lucy's world as well. He could use this newfound power to try and take over both the Bleach universe and Lucy's world. His cunning and powerful abilities would make him a formidable foe for Lucy and the Bleach characters to face.
A time - traveling bounty hunter could be the main antagonist. He might be after Lucy because she has somehow disrupted the time - space continuum. He's cold - hearted and will stop at nothing to bring her to some time - traveling justice.
It could be a dark wizard who wants to use the baby's power for his own evil deeds. Maybe the baby has some special magic that the wizard thinks can help him take over the magical world.
It could be another powerful celestial dragon who opposes Lucy's ideas or presence.
Well, in a Loud House Lori vampire fanfiction, the plot might start with Lori being lured into an old, spooky part of town where she gets bitten by a vampire. After that, she has a whole new set of problems. Her siblings start to notice her odd behavior, like being awake all night. She has to deal with her new powers, such as being able to run extremely fast. She could also be in a situation where she has to protect her family from other vampires who see her as a threat because she doesn't want to join their dark ways. This would lead to a series of adventures and challenges for Lori as she tries to balance her two lives.
Maybe a time - traveling collector. This character travels through different times collecting rare items or memories. When Lincoln enters a particular time period, the collector sees Lincoln's unique family situation as a valuable item to collect and tries to keep him trapped in that time, away from his own family.