One idea could be a story where Wally and Barry team up to face a new, extremely fast villain. They have to combine their speedster powers in a unique way to defeat the bad guy.
First, you should think about the relationship between Wally and Barry. Are they friends, rivals, or something in between? Then, come up with a basic plot idea, like a mission they go on together or a problem they need to solve.
You can start by looking on fanfiction.net. It has a large collection of fan - created stories, and you can easily search for barry allen x wally west fanfiction there.
Another interesting one is 'Two Flashes, One Destiny'. This fanfiction takes place in an alternate universe where Barry and Wally have to figure out a way to merge their powers to save the world. It's full of action - packed scenes and emotional moments as they struggle to understand each other's strengths and weaknesses.
A common theme is friendship. Barry and Wally are often shown having a deep bond and relying on each other. Another theme is heroism. They both are heroes and the fics usually revolve around them saving the day.
The character dynamics. Barry's speed and his heroic nature, combined with Wally's own charm and Jinx's chaotic side can create really engaging scenarios. For instance, a fanfic where Jinx's powers cause trouble and Barry and Wally have to work together to stop it while also trying to understand her motives.
One great 'Barry Allen and Wally West Fanfic' is 'Speedsters United'. It shows their relationship as they team up to face a new speed - stealing villain. The story does a wonderful job of highlighting their different personalities within the Flash family.
Obviously, Barry and Wally are the most popular. Barry, as the more experienced speedster, often serves as a mentor figure. Wally, with his own unique charm and potential, is also a favorite. His growth and development in the fanfiction are usually very interesting to read about.