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.
If it's a crossover with 'Frozen', Elsa and Anna would be great characters. Ariel could meet them when a magical iceberg drifts into her underwater world. Elsa's ice powers and Ariel's water powers could interact in interesting ways.
Well, if you're doing a crossover with 'Buffy the Vampire Slayer', Buffy herself would be a great addition. Her slayer skills would create an interesting dynamic with the vampires from 'Interview with a Vampire'. Also, Angel from 'Buffy' could be included as he has a similar vampire - with - a - soul situation. Another good character could be Van Helsing from the various Van Helsing stories. His vampire - hunting skills would bring a lot of action to the crossover.
Another option could be Van Helsing from the movie of the same name. He is a classic vampire hunter. He and D could exchange hunting techniques and knowledge. And they could form an alliance when they discover a group of vampires that are planning something really big, like taking over a major city.
Hermione Granger from 'Harry Potter' could be an interesting addition. She could use her magic knowledge to study the mummy. Maybe she could find a spell to control or at least understand the mummy better. It would be a really unique combination of magic and ancient Egyptian mystery.
Professor Oak could also be an interesting addition. As a Pokémon expert, he might be fascinated by Mewtwo and try to study it. This could create a conflict as Mewtwo is not one to be easily studied. It could also lead to a deeper understanding of Mewtwo's origin and abilities.
Batman would be a great character to include. His no - kill rule contrasts with Deadshot's assassin nature. Their interactions would be full of tension and moral debates. Harley Quinn could also be included. Her chaotic personality could add an element of unpredictability to the story. She might team up with Deadshot for a mission or be a distraction during one of his hits.
Alucard from "Hellsing" would be a great addition. His overpowered nature and complex personality would contrast well with D's stoicism. They could have an interesting dynamic, either as allies or enemies depending on the plot.
For a crossover with a classic cartoon like Tom and Jerry, Jerry could be a really interesting character to include. Jerry is always getting into scrapes with Tom. Maybe Doc McStuffins could be the one to treat Jerry's little bumps and bruises after his run - ins with Tom. And she could also try to make Tom understand that he shouldn't be so rough with Jerry.
In a 'baki the grappler crossover fanfiction', characters like Kenshiro from 'Fist of the North Star' would be great. His powerful martial arts skills would contrast well with Baki's grappling. Also, Goku from 'Dragon Ball' with his superhuman strength and energy attacks. And Naruto from 'Naruto' with his ninja skills and large - scale jutsu could create interesting interactions with Baki.
The Teenage Mutant Ninja Turtles would be great. Leonardo, Donatello, Michelangelo, and Raphael could meet Kick and teach him some new fighting moves. In return, Kick could show them some crazy skateboarding tricks. They could all team up to fight a new villain in New York City.