Another possible storyline is that a villain from the Flash's world somehow ends up in the Glee universe and starts causing trouble. The Glee club members, with the help of the Flash (or someone with Flash - like powers), have to come together to defeat the villain. They could use their unique skills, like singing in harmony to disrupt the villain's powers or something equally creative.
Well, there could be a story where Flash is a secret fan of Glee. He watches their shows in his downtime between fighting villains. And then, one day, the Glee club members find out about him and they ask him to use his powers to promote their club. It could lead to a really fun and unique story of cross - over adventures.
One idea could be that the characters from Glee find themselves in Central City during one of The Flash's battles. Maybe they get caught in the crossfire and The Flash has to save them while also dealing with the meta - human threat. For example, Rachel could be singing on the street when a meta - human causes chaos nearby.
Another concept is that a villain from The Flash's world uses the power of music (inspired by the Glee club's performances) to create chaos. The Flash teams up with the Glee club to reverse the effects of the music - based villainy. The Glee kids could use their knowledge of harmony and rhythm to counter the negative musical energy, while the Flash uses his speed to stop the physical manifestations of the villain's attacks.
One interesting storyline could be that the Glee characters end up in the Pokemon world. They could form a Pokemon performance troupe. For example, Rachel could be a trainer who uses singing to boost her Pokemon's power during battles. Finn might be more into the physical aspects of Pokemon training, like competing in Pokemon athletics. Another idea is that the Pokemon themselves could have musical abilities and they join the Glee club in a special Pokemon - human co - existing world.
In some fanfictions, there might be a situation where the Flash's speed force is somehow connected to the supernatural world in Supernatural. Maybe it attracts the attention of angels or demons who want to use it for their own purposes. This could lead to a complex plot where the Flash has to deal with both his own villains and the otherworldly beings from Supernatural.
One idea could be to have the characters from Glee perform at a Central City event in the Flash universe. They could sing a song to boost the morale of the citizens during a time of crisis.
Perhaps there could be a story where the 'glee' club members visit the Irwin's zoo. They get to see all the amazing animals and end up learning a lot about wildlife. In the process, they might also teach the Irwins some new music and performance styles.
One common theme is the characters from both shows coming together to solve a problem. For example, the Flash characters might use their superpowers to help the Glee club win a big competition that's being threatened by some sort of villain.
One plot idea could be that a character from Flash, like Barry Allen, somehow gets zapped into the Glee universe. He could be confused by all the singing and dancing at first, but then use his speed to help the Glee club prepare for a big competition in a very short time. For example, he could quickly gather all the necessary props or help the members with their choreography by demonstrating moves at super - speed.
One interesting Glee and The Flash fanfiction could be about a character from Glee suddenly getting super - speed like The Flash. They could use this new power to perform amazing musical numbers in a whole new way, like running across the stage so fast that it creates a cool visual effect while singing.