There could be a 'Dark Barry Allen fanfiction' that delves into an alternate universe where Barry is corrupted by a powerful enemy. In this story, he starts to question his own morals and values, and his friends have to find a way to bring him back to the light. This type of fanfiction often explores the darker side of his character, like his potential for vengeance and how it can change him.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for 'Dark Barry Allen' within its search bar. There are likely many different interpretations of a dark Barry Allen there.
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.
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.
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.