One common theme is the clash of different vampire cultures. In Twilight, vampires are more secretive and have their own set of rules, while in House of Night, vampyres are more open and have a different social structure. Crossover fanfictions often explore how these two cultures would interact when they meet.
One idea could be to cross it with a vampire hunter story. Have the characters from House of Night face off against a group of vampire hunters who don't understand their different kind of vampirism. It would create a lot of action and drama.
Aphrodite from House of Night is also a great character. She's complex and often causes trouble. In a crossover, she could bring her mischievous side to a different setting. Let's say it's a crossover with a fairytale world. Aphrodite might be the one to manipulate the fairytale characters for her own gain, adding an interesting layer of conflict.
One great 'Twilight House of Night Crossover Fanfiction' is 'The Merging Worlds' on Fanfiction.net. It has an interesting plot where the characters from both universes meet in a small town. The author does a good job of combining the vampire lore from Twilight and House of Night.
One idea could be to have the characters from 'House' end up in the world of 'Fruits Basket'. Maybe House could try to diagnose the curse that plagues the Sohma family. His rational and often brusque approach would clash with the more emotional and superstitious aspects of the 'Fruits Basket' world.
In this crossover fanfiction, the Hulk's strength could be used to help the Loud family in a big way. For example, there could be a natural disaster about to hit Royal Woods, like a huge flood or an earthquake. The Hulk, with his superhuman strength, could hold back the floodwaters or stop the shaking ground. Meanwhile, the Loud kids could be helping the townspeople evacuate. It would be a story of how two different universes come together to save a town.
One idea could be that the Loud family somehow gets transported to Gotham City. Lincoln could befriend Robin and they team up to solve a mystery that involves both Gotham and Royal Woods' villains. Maybe the Joker has a plan that involves stealing a special item from Royal Woods, and the Louds have to work with Batman to stop him.
Well, for a fanfiction about Hermione and house - elves on prank night, it could start with Hermione coming up with the idea to have some fun with the elves. The elves, being magical creatures, could use their unique abilities in the pranks. They might use their magic to make things levitate or change colors in the Gryffindor common room. Hermione would be there to keep things in check and make sure no one gets too carried away.
Zoey Redbird is often a very popular character. She is the main character in the original series, and in fanfiction, people like to explore her story further, maybe give her different challenges or different romantic interests.
There could be a fanfiction where a group of students form an underground club in the House of Night. They explore forgotten parts of the school's history and in the process, discover a hidden prophecy that they believe is related to their future. As they try to decipher it, they face opposition from the school authorities who think they are up to no good.