The girl they are both interested in is also a great character to focus on. You can develop her character more, give her more agency and perhaps some hidden talents or secrets that would surprise the guys.
Definitely the two male leads. Their competitive and comical relationship is the heart of the movie and can be explored further in fanfic. You can show more of their backstory and how they became such rivals.
Philip and Elizabeth Jennings are obvious choices. Their complex relationship and double lives are full of potential for fanfic exploration. You can create scenarios that further test their loyalty to each other and to their country.
Jack is a great character to focus on. He's the main character, so there's a lot of material to work with. His growth in martial arts and his relationships with the other characters are rich areas to explore. For example, you could write about his struggle to master a new move and how his friends help him.
I think focusing on the Wattersons family as a whole can be really interesting in a Gumball fanfic. You can show their family dynamics, like how Richard's laziness affects the household, or how Nicole's strictness is both a help and a hindrance. Then there are the friends like Carrie, the ghost girl. Her spooky nature and her friendship with Gumball and the others can add a unique element to the story. And Tobias, with his one - sided crush on Penny, can also bring in some elements of unrequited love and jealousy into the fanfic.
The main characters could include some of the new generation as well. For example, Albus Potter or Scorpius Malfoy. They could be at the center of this 'war' in the fanfic, perhaps dealing with the consequences of their parents' actions. Ginny Weasley could also be a main character, as she's a strong witch in her own right. And then there are the Hogwarts professors. Professor McGonagall could be leading the defense of Hogwarts in this war - like situation, and Professor Flitwick could be using his magical expertise to help the side he's on.
Chika Fujiwara is also a popular character to focus on. Her bubbly and sometimes clueless personality can add a lot of humor to the story. You can write about her accidentally getting in the middle of Kaguya and Miyuki's relationship, or even having her own love - related subplots.
Austin and Ally are the main characters, so they should be at the center. However, Dez can't be ignored. His wacky ideas for Austin's music videos and his general goofiness can create some hilarious scenarios in a fanfic. You could also explore more of the relationship between Dez and Trish, which is often overlooked in the show.
Jack Frost is a very popular choice. His mysterious past and his cool powers make him an interesting character to explore. You can dig into his feelings of isolation before becoming a Guardian and how he's changed since then.
Fat Amy is a great character to focus on. Her larger - than - life personality and unique sense of humor can bring a lot of fun to a fanfic. You could write about her secret talents or her family back in Australia.
Nanisca is an obvious choice. She is the central figure in the movie, so there is a lot of material to work with. You could explore her leadership further, her past traumas, and how she continues to grow as a leader.
Definitely the main character's closest friends or allies. Their relationships can add depth and emotion to the fanfic. Also, antagonists are important. A well - developed villain can create conflict and tension. You can explore their motives and make them more than just a one - dimensional 'bad guy'. And don't forget about those characters who seem minor at first but have hidden potential or secrets.