Maybe Elijah from 'The Originals' could be the main character in this crossover fanfiction. He has a noble and complex personality. He could be the one who discovers the vampire from another realm or storyline and tries to either help or control the vampire depending on how he perceives the situation.
It could be a new vampire character that is created specifically for this crossover. This vampire could be a hybrid of some sort, half - vampire and half - something else, and their unique nature draws the attention of both the vampire world and the characters from 'The Originals'.
One possible plot could be that a vampire gets caught in the middle of the events in 'The Originals'. Maybe the vampire is an ancient one who has some secrets about the origin of the vampire species that the Mikaelsons are desperate to know. This could lead to an alliance of sorts between the vampire and some of the Mikaelsons while others see the vampire as a threat.
Luffy could be an interesting main character in a vampire - themed One Piece fanfiction. Imagine if he was turned into a vampire but still had his rubber powers. He would have to deal with his new vampiric nature while leading his crew.
Perhaps an anti - hero could be the main character. They find the Infinity Armor and initially use it for their own self - serving purposes. But as the story progresses, they start to see the bigger picture and become more heroic, using the armor to protect not just themselves but also the different worlds they encounter during the crossover.
There could be an external force from the slime's original world that is trying to bring it back by any means necessary. This force could be very powerful and not care about the consequences in the Rosario Vampire world. They could be the ones behind some of the strange events that start to happen in the school, like summoning dark creatures to attack the school in order to force the slime to reveal itself so they can capture it and take it back.
Kakashi could also work well. His laid - back yet highly skilled personality would be a contrast to the more mysterious and often dark nature of a vampire. As a vampire servant, he could use his vast knowledge of ninja techniques in combination with his new vampiric powers. For example, he could use his vampire - enhanced speed to perform his Raikiri even faster and more powerfully, while also having to deal with the thirst for blood and how it affects his relationships with his comrades.
Draco Malfoy and Kaname Kuran could be an interesting pair. Draco is from a wealthy and powerful pure - blood wizard family, and Kaname is a powerful pure - blood vampire. They both have an air of superiority about them. It would be interesting to see how they interact, whether they become rivals or somehow find common ground. Maybe Draco would be intrigued by Kaname's vampiric powers and try to use them for his own gain, or Kaname might see Draco as a useful ally in his own plans.
Goku could be an interesting main character in a vampire fanfiction. Imagine Goku having to deal with the newfound thirst for blood while still trying to protect the Earth. His strong moral compass would be put to the test as he battles against his vampire instincts.
It's likely that Ben with the Omnitrix would be one of the most powerful. He has access to a wide variety of alien forms, each with their own unique abilities. For example, Heatblast could deal with cold - based threats in the Rosario Vampire world, and XLR8 could outrun any fast - moving vampires or enemies. However, some of the powerful vampires like Alucard in Rosario Vampire also have incredible strength, speed, and magical abilities. So it could be a close call between Ben in his most powerful alien forms and the top - tier vampires.
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.
Another option is that a new character introduced in the crossover could be the main character. For instance, in a Naruto - Fullmetal Alchemist crossover, an alchemist from the FMA world could stumble upon Naruto's sealed state and make it their mission to free him, while also getting involved in the politics and power struggles of the Naruto world.