A possible plot could be that Barry Allen is running through time and space and accidentally disrupts the Glee reunion. He then feels responsible and decides to help make it the best reunion ever. He could use his super speed to bring in some of the old Glee members who couldn't make it initially. And in the process, the Glee characters teach him some valuable lessons about the power of music and friendship.
If Barry Allen joined the Glee reunion in fanfiction, he might bring a new perspective to the group. His experience of saving the world and facing challenges could inspire the Glee characters to be more confident and take risks in their performances. Also, he could use his superpowers to create some amazing visual effects during their shows, like running around the stage so fast that he creates a cool blur effect.
Barry Allen x Savitar fanfiction often plays with the idea of alternate universes. Writers may create scenarios where they are not enemies but rather allies or even something more. This can lead to interesting character developments. For example, Savitar's time - related powers combined with Barry's speed could be used in unique ways to solve problems or face new threats that were not present in the canon storyline.
One idea could be that the Krypton OC has some unique powers due to their Kryptonian heritage that Barry Allen initially doesn't understand. For example, they might be able to manipulate a different form of energy that interacts strangely with the Speed Force. This could lead to a series of adventures where they have to figure out how to work together without their powers interfering with each other.
Barry Allen's death in the comics is a complex situation. Sometimes he does die, but often there are plot twists and resurrections as part of the ongoing comic narrative. It all depends on the creative direction of the writers and the demands of the story at that time.