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.
Another theme is the exploration of their different personalities. Caitlyn is often seen as more refined and proper, while Vi is more rough - and - tumble. Fanfics play with how these differences can create both conflict and attraction. For instance, their different approaches to solving problems in Piltover can lead to interesting interactions and storylines.
Fanfiction.net is also a good place. There are many different stories about them there. However, the quality may vary, so you might need to do some sorting. But it's definitely worth checking out.
One great Caitlyn and Vi fanfiction could be 'The Reconciliation'. It focuses on their relationship after a big fight in Piltover. The story shows how they overcome their differences and work together again.
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.
There's also 'Vi and Caitlyn: A New Beginning'. This fanfiction focuses on how they might start to build a life together after the chaos in Piltover. It has some really sweet moments between the two characters.