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.
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.
One idea could be a story where the Loud siblings discover a hidden attic in their house that leads to a magical world. Another could be a fanfiction focusing on Lincoln's adventures in a parallel universe where he has different superpowers. Maybe a story about the sisters teaming up to start their own band.
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.
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.
One idea could be a crossover where the Loud family members gain ninja abilities. For example, Lincoln could be a skilled shadow clone user like Naruto. Another could be that the Naruto characters end up in the Loud House world and have to adapt to the strange family dynamics there. Maybe Naruto tries to teach the Loud siblings some of his ninja values like perseverance.
One idea could be a crossover where Naruto characters end up in the Loud House world. For example, Naruto and his friends could attend the Loud siblings' school and get involved in their daily shenanigans like dealing with bullies or school competitions. Another idea is a story where the Loud House kids discover ninja powers similar to those in Naruto. They could start training and use these powers to solve problems in their town.
One idea could be exploring a different timeline where the Loud siblings have different careers from a young age. For example, Lincoln could be a successful young artist. Another idea is to focus on a mystery within the Loud house, like a hidden room with a family secret. Or you could do a fanfiction where the family goes on an adventure to a magical world that only they can access.
One interesting plot could be the vampire sisters trying to fit into the Loud House family while hiding their true nature. Maybe they have to deal with the chaos of the Loud siblings while also craving blood in secret.
One idea could be to have the characters from Helluva Boss somehow end up in the world of The Loud House. Maybe Blitzo and his crew are on a mission and accidentally cross dimensions. Another idea is a crossover where the Loud siblings get involved in the chaotic business of I.M.P. from Helluva Boss.
One idea could be to have Lincoln as a young Jedi in training. He could be learning from an older character, like maybe an alien version of his dad. Another could be a story where the Loud sisters are part of a rebel alliance in the Star Wars universe, fighting against the evil Empire. Maybe Leni could be a fashion - conscious rebel, using her unique style to create disguises.