Kim Possible herself. She's a strong, independent character. In a fanfiction with Danny Phantom, she could be the one to teach Danny a thing or two about being a hero without superpowers (at least in the traditional sense). Her determination and resourcefulness would be a great contrast to Danny's more impulsive use of his powers at times. Plus, seeing her react to the supernatural elements of Danny's world would be fascinating.
I think Ron Stoppable could be an interesting focus. He's not as powerful as Kim or Danny, but his insecurities and his growth as a character could be explored well in a crossover fanfiction. He could be the one who has to step up in a big way when Danny and Kim are dealing with a threat that requires a different kind of thinking. His relationship with Kim could also be affected by Danny's presence, adding an extra layer of drama.
Well, Danny Phantom has the ability to phase through objects and fly at high speeds, which is really cool. Ben 10 can transform into so many different aliens, each with their own unique powers. And Kim Possible is super athletic and has all those useful gadgets. However, I think Ben 10 might be the most powerful in a crossover. His Omnitrix gives him access to a wide variety of powers that can be used in different situations. For example, if they were in a fight underwater, he could transform into an alien that is adapted to aquatic environments. But again, it's all about how the story is written and what challenges they face.
Vlad Masters could be a likely antagonist. He might see Kim as a threat to his plans regarding Danny. Since he's always looking for ways to get Danny on his side or out of the way, he could try to use Kim as a pawn. Maybe he tries to manipulate Kim into thinking Danny is the bad guy, or he uses his ghost powers and wealth to create problems for both of them.
Sam and Tucker are really interesting characters to focus on. Sam's gothic personality and her strong beliefs can lead to some great storylines. Tucker's tech - savvy nature can be used in many ways, like developing new gadgets or helping Danny figure out some tech - related mysteries. And Jazz, Danny's sister, with her psychology knowledge could play a crucial role in understanding Danny's situation better in a fanfiction.
Danny himself is always a great focus. His dual identity as a normal teen and a ghost - fighting hero gives a lot of room for exploration in family fanfiction. His parents are also very interesting, especially their reactions to Danny's powers and how they could grow and change in a family - centered story.
Danny and Kim are often paired up. Their different skill sets make for an exciting combination. Danny with his ghost - fighting abilities and Kim with her acrobatic fighting skills and access to various gadgets.
One interesting crossover could be Danny and Kim teaming up to fight a common supernatural enemy. Maybe a villain that has both magical powers and high - tech gadgets at their disposal. Danny's ghost powers would complement Kim's martial arts and gadget skills.
Definitely Harry Potter and Kim Possible themselves. Harry has his whole wizarding world experience and his fight against dark magic, and Kim has her crime - fighting skills and high - tech gadgets. Their different worlds and abilities would create a really interesting dynamic as they try to understand each other and work together.
One idea could be that Danny and Kim have to team up to stop a new villain who has the power to manipulate time. This would put both their skills to the test as they race against the clock. Another plot could involve a ghostly artifact that Kim discovers during one of her missions, and she has to call on Danny's expertise in the supernatural to figure out what it is and how to deal with it.
One interesting crossover could be the characters teaming up to fight a common enemy. For example, Danny Phantom's ghost - fighting abilities, Ben 10's alien forms and Kim Possible's martial arts and problem - solving skills could be combined. They might face a powerful villain that neither could defeat alone.
The Fairly OddParents' Timmy Turner could be a fun character to include. He has his own set of magical fairy godparents. Imagine the chaos that could ensue when Danny's ghost powers and Timmy's fairy - granted wishes interact. They could accidentally create some really wild situations as they try to solve a problem together in a shared Nickelodeon universe.