I think focusing on Ruby and Weiss would be good. Ruby as the leader has so many aspects to explore, like her insecurities and how she overcomes them. Weiss, on the other hand, has her own journey of breaking free from her family's expectations. They also have a great dynamic together, which can be developed further in the fanfic. But Blake and Yang are also important, especially if you want to show their friendship and how they support each other through tough times.
Ruby is always a great choice. Her journey from a young huntress - in - training to a more experienced leader provides a lot of material for fanfiction. You can explore her growth, her relationships with her team, and how she copes with the increasing challenges.
Brennan and Booth are obvious choices. Their relationship is at the core of the show, so exploring different aspects of it in fanfiction can be really interesting. You can dig deeper into their pasts, like Brennan's childhood traumas and how that affects her relationship with Booth, or Booth's military experiences and how they shape his views on love and commitment.
I think Weiss from RWBY could be really interesting to focus on. Her strict upbringing and powerful semblance would contrast well with the more chaotic nature of the Agents of Mayhem. From the Agents of Mayhem, Brick would be a good character. His brute strength and no - nonsense attitude could lead to some humorous and action - packed scenes when paired with the RWBY characters.
Ruby is always a great choice. In the Mirrorverse, her innocence could be twisted in an interesting way, or she could be the one to bring light to a very dark Mirrorverse. Weiss is also interesting as her family's influence might be different, and she could have a completely different set of skills or abilities. Blake could have a unique story too, perhaps her faunus heritage is more pronounced or has a different meaning in the Mirrorverse.
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.
A great plot for 'rwby only us fanfic' could be a competition within Beacon. But this isn't just any competition. It's a test of not only combat skills but also their knowledge of the history of Remnant. RWBY could face challenges that require them to work together and also make difficult decisions. Another idea is that a strange energy wave hits Vale and starts affecting the people's memories. RWBY has to find the source of this wave and stop it before everyone forgets who they are and what they're fighting for.
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.