Perhaps a plot where The Flash crosses over with characters from Legends of Tomorrow to prevent a time - related disaster. They could use their different abilities, like The Flash's super - speed and the Legends' knowledge of different time periods and artifacts. And a scenario where a new villain uses technology from different CW shows against The Flash and his crossover allies would create a great story.
One idea could be a crossover where The Flash teams up with characters from other CW shows to stop a multiverse - threatening villain. For example, bringing in characters from Arrow to help with their combat skills and strategic planning. Another could be a time - travel plot where The Flash accidentally travels to an alternate version of the CW universe and has to find his way back while dealing with the differences in that world.
A cool plot could involve a magical threat that requires The Flash to team up with characters from Supernatural. They could combine their powers and knowledge, like The Flash using his speed to help the Winchester brothers get to different locations quickly during their hunts. Also, a plot where The Flash's speed causes some sort of rift in the CW universe that starts merging different shows' storylines together would be very interesting.
One idea could be a crossover with another superhero show where The Flash has to team up with their heroes to stop a multiverse - threatening villain. Maybe he teams up with Arrow and they have to combine their skills to take on a speedster from another dimension who can also turn invisible.
One idea could be that the characters from the CW shows end up in the Beauty and the Beast world. For example, a group of CW superheroes might get transported there and have to navigate the magical and dangerous world while also trying to find a way back home. Another idea is that the Beast could be a character from a CW show who is cursed in a similar way and the Beauty character has to come from another CW show to break the curse. It could also be interesting if the two worlds merge and characters from both have to learn to co - exist and deal with the cultural differences between the CW universe and the Beauty and the Beast universe.
In a fanfiction, there could be an alien race that is immune to Supergirl's powers but is vulnerable to the speed force. Flash discovers this during a battle where Supergirl is struggling. He then uses his speed to create a barrier around the aliens, trapping them long enough for Supergirl to figure out a new way to defeat them, like using her super - breath in a new way that Flash helps her direct with his speed.
One idea could be that the Flash and Arrow team up to stop a meta - human who has the ability to control time. They have to race against the clock (literally) to figure out how to neutralize the villain's powers before he rewrites history in a way that destroys Central City and Starling City.
One idea could be that a powerful villain creates a time - space rift that pulls both Supergirl and Flash into an alternate dimension where they have to team up to find a way back home. Their powers work differently in this new dimension, and they have to learn to adapt quickly.
One idea could be that a strange meta - human from The Flash's world somehow ends up in Riverdale. This meta - human has the power to manipulate memories, and starts causing chaos by making the Riverdale characters forget important things about each other. Archie and his friends then team up with Barry Allen to stop this meta - human before their town is torn apart.
One idea could be that the Winchesters are hunting a meta - human in Central City who has powers similar to a demon. The Flash team tries to stop them from killing the meta - human, thinking they are just bad guys. Another could be a time - travel situation where Barry accidentally travels to a world where the Supernatural events are real and has to team up with Sam and Dean to get back home. A third idea is that a powerful witch from the Supernatural universe uses her magic to create chaos in Central City, and the Flash and his friends need the help of the Winchesters to defeat her.
Maybe there's a time - traveling mishap that brings Flash and Supergirl to an alternate universe where their powers are reversed. Flash has to deal with super strength and heat vision, while Supergirl has to learn to control super speed. They must figure out how to get back to their own universes and in the process, they bond over their shared confusion and determination.
Perhaps there's a villain in Gotham who has discovered a way to siphon the Flash's speed force. The Flash has to go to Gotham to stop this villain, and in the process, he meets the various characters of Gotham like Gordon and Catwoman. They all have to work together to not only stop the villain from stealing the speed force but also from using it for some evil plan in Gotham.
A cool plot might be that a new meta - human emerges who has the power to steal the abilities of other heroes. This meta - human targets the Flash first, leaving him powerless. Batman then has to take the Flash under his wing and train him in more traditional combat skills while they figure out a way to get the Flash's powers back. Along the way, they discover that the mastermind behind this meta - human is an old enemy of Batman who wants to use the Flash's powers to carry out a grand scheme in Gotham.