I think Rise from the Investigation Team would be fascinating. Her fame as an idol and her hidden insecurities could play into the story when she meets the Phantom Thieves. From the Phantom Thieves, Makoto would be interesting. Her strict sense of justice could be at odds with the Investigation Team's more flexible view of the law in some situations, leading to some intense character moments.
Definitely Yu from the Investigation Team. His ability to form strong bonds with others and his leadership skills could make for some really interesting interactions with the Phantom Thieves. On the Phantom Thieves' side, Joker is a must - focus. His calm and strategic mind would contrast well with Yu's more empathetic approach. Their clashes and potential understanding of each other would be great to explore.
Well, the plot could involve a clash of ideologies. The Investigation Team might see the Phantom Thieves' methods as too radical. For example, the Investigation Team, with their focus on traditional detective work and upholding the law, could be tasked with stopping the Phantom Thieves who are stealing hearts to change society in their own way. Maybe there would be a character from each side who starts to question their own group's actions after getting to know the other side better.
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.
Jase Ballenger is often a popular choice. His complex character, being a thief leader, offers a lot of scope for fanfiction. Writers can explore his leadership decisions and his inner thoughts.
Joker is often very popular in fanfiction. As the leader of the Phantom Thieves, he has a lot of mystery and charisma. Writers like to explore his backstory further or put him in different, challenging situations where he has to use his wits and Persona powers in new ways.
Miss Martian from Young Justice could be a great focus. Her powers of telepathy and shape - shifting make her a formidable opponent. Also, her experiences as a young hero trying to fit in and be accepted, especially when facing the more established Justice League, can lead to some really interesting character development and plot twists.
Hannibal is often a popular focus. His leadership skills and strategic mind make for great storylines. Writers can explore his past military experiences in more detail or create scenarios where he has to outwit an even smarter adversary.
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.
Madame Giry can be an interesting character to focus on as well. She has a connection to the Phantom and could be used to explore the backstory of the Phantom and the opera house in a different way in fanfiction. For example, a fanfic could explore her own past relationship with the Phantom or her role in protecting him.
Christine is also a central figure. Fanfiction can explore her feelings for the Phantom and Raoul in more depth. Maybe she has a secret side that she hides from both of them, or she could be given more agency in the story, rather than just being the object of the men's affections.