Definitely Harry Potter and Danny Phantom themselves. They are the main characters of their respective universes, so it's natural for them to be the focus in crossovers. Fans love to imagine how they would interact and what kind of adventures they would have together.
Hermione Granger is also quite popular in these crossovers. Her intelligence and knowledge of magic could be really useful when combined with Danny's powers. She might be the one to figure out how to use Danny's ghostly abilities in a more strategic way within the wizarding world. Another popular character is Danny's best friend Tucker. His tech - savvy nature could bring a new perspective to the magical world of Harry Potter.
Some fanfictions also like to pair Danny's friends, like Tucker or Sam, with the Hogwarts students. For example, Tucker might get along well with a tech - savvy wizard at Hogwarts. They could work together on some sort of project that combines muggle technology and magic. Sam, with her strong personality, could be paired with a character like Hermione. They could bond over their love for knowledge and their determination to fight for what they believe in.
I think pairing Danny with Hermione would be great. Hermione is really smart and she could be fascinated by Danny's ghost powers. She might try to figure out the scientific or magical explanation behind them. Danny, on the other hand, could learn a lot from Hermione about magic, and she could help him understand the wizarding world better. For example, she could teach him some basic spells that could be useful when combined with his ghost powers.
Obviously, Danny and Dan themselves are very popular. Danny as the main hero and Dan as his dark and complex counterpart are central to many stories. Jazz, Danny's sister, is also popular as she can provide a different perspective and often gets involved in Danny's ghost - fighting adventures in fanfiction.
One popular theme is the clash of cultures. The modern - day, American - based Danny Phantom world meeting the traditional, British wizarding world of Harry Potter. Another is the power exchange, like wizards trying to understand and maybe even use Danny's ghost powers. And of course, friendship between the characters from both universes is a big theme.
Danny himself is often a very popular character. After all, he's the main character of the original series, so in fanfiction, his adventures are usually central. Sam and Tucker are also quite popular as they are Danny's closest friends. Their relationships with Danny and how they support him in different situations are often explored in fanfiction.
Sam and Tucker are also quite popular. In the multiverse fanfiction, they can have different roles. Maybe in one story, Sam has some hidden ghost powers of her own and becomes a key player in a multiverse adventure. Tucker could be a tech - genius in one universe and a powerful magician in another, adding new dimensions to their characters.
Wolverine is also a very popular character in these fanfictions. His tough - guy attitude and his claws and healing factor are really cool elements. Fans love to see how he reacts to Danny's world of ghosts and the different situations they get into together.
Sakura from Naruto might also be popular in these fanfictions. She could potentially have some interesting interactions with Danny, like comparing her medical ninjutsu with Danny's knowledge of ectoplasm - based healing (if there is such a thing in the crossover). Also, Tucker from Danny Phantom might be a popular character as he could be the tech - savvy guy in the Naruto world, coming up with new inventions using a combination of Naruto - world materials and his own tech knowledge.
Danny himself is obviously one of the most popular characters. Since the fanfiction is about his college years, readers are interested in seeing how he has grown and how he will handle new challenges. Tucker is also popular as he is Danny's best friend and can provide some tech - savvy support. Sam is another beloved character as she has always been by Danny's side and her strong - willed personality often adds interesting dynamics to the stories.
Another popular pairing is Danny with Deadpool. Deadpool's crazy antics and Danny's ghostly nature would make for a hilarious combination. Deadpool might be constantly trying to make Danny break his cool, calm ghostly demeanor. And Danny would have to deal with Deadpool's fourth - wall - breaking tendencies.