One common theme is friendship development. Since they come from different backgrounds in the Arcane universe, their journey to becoming friends or even closer is often explored.
A great fanfiction idea is one that delves into their relationship dynamics. For example, how they deal with their differences in social status. Caitlyn being from the upper class of Piltover and Vi from the undercity. It could show how they overcome these barriers and grow closer through shared experiences and mutual respect.
A common theme is also the exploration of their backstories. How Vi grew up in the undercity and Caitlyn in Piltover, and how these different backgrounds shape their characters and interactions. Some fanfics also focus on the power dynamics between them, considering their different skills and positions in society.
Fanfiction.net is also a good place. There are many writers who contribute to the Arcane fandom there. However, the quality may vary, so you might need to skim through a few stories to find the really good ones about Caitlyn and Vi.
You can try fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfics, and you can search for Arcane Vi and Caitlyn stories specifically. Just use the search bar and type in relevant keywords.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are tons of Vi and Caitlyn fanfictions there. Just search for their names in the search bar, and you'll get a long list of stories. Another option is FanFiction.net, though AO3 generally has a more organized tagging system for Arcane related fanfics.
One idea could be a story where Vi and Caitlyn have to team up to solve a mystery in the undercity that's been causing chaos. They start off with their differences but gradually learn to trust and rely on each other more as they dig deeper into the case.
Romance is also a very popular theme. Given their relationship in the Arcane series, fanfiction writers like to explore the romantic aspect between them. This can range from a slow - burning love story to more passionate and intense relationships. Some fanfictions focus on how they overcome their differences and fall in love.
Another theme is justice. Since both are characters associated with law and order in their original setting. Fanfics often explore how they uphold justice in different situations, sometimes even having to make tough choices between what is legal and what is morally right.
A plotline could be that there's a new criminal organization in Piltover that's using advanced technology to commit crimes. Vi and Caitlyn are assigned to take it down. Along the way, they face many challenges and obstacles that test their skills and their relationship. As they get closer to the mastermind behind the organization, they also discover some secrets about the city's power structure that could change everything.