Well, in fanfiction where Barry Allen turns into Black Flash, it's a major transformation. Firstly, his powers could become more unpredictable and dangerous. He might not be able to control his speed as he used to. Secondly, his friends like Iris, Cisco, and Caitlin would be horrified. They would try to find a way to bring back the old Barry. Emotionally, Barry would be in turmoil as the Black Flash entity takes over. His speedster friends from other cities like Wally and Jay might also be in danger as the Black Flash is known to target speedsters. It would be a very different and much darker story for the Flash universe.
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.
If Naruto becomes the Flash in fanfiction, he would be incredibly fast. He could use his new speed to outmaneuver his enemies in combat. For example, in battles against the Akatsuki, he could zip around and strike them before they even realize it. His Rasengan might also be enhanced as he could deliver it with the speed of the Flash, making it even more powerful and difficult to dodge.
One of the best is 'The Flash: Rebirth'. It delved deep into Barry Allen's return and his complex relationship with the Speed Force. It also reintroduced many elements of his character and his place in the DC Universe.
The plot would become much darker. Instead of the usual heroics of Barry Allen, there would be chaos. His enemies might initially be confused and then try to use his new dark state to their advantage.
If Harry Potter becomes the Flash in fanfiction, he might use his new speed powers to quickly get around Hogwarts. He could be the first to reach the library for research books or be the fastest on the Quidditch pitch, leaving everyone in awe.
Barry Allen became The Flash after being struck by lightning while working in his lab. The lightning was charged with chemicals, which altered his body's chemistry and gave him super - speed. This event transformed him into the Scarlet Speedster we know as The Flash.
One possible scenario could be that Barry Allen is exposed to some dark energy or a time - related anomaly that slowly starts to corrupt his mind and make him turn into Savitar. This could be due to his overuse of the Speed Force, which has unpredictable effects. Maybe he starts to see the world from Savitar's perspective, believing that the only way to 'fix' things is through Savitar's extreme methods.
Barry Allen becomes Flash when he's exposed to a lightning strike and chemicals in his lab. This accident gives him super-speed powers.
In this fanfiction, Barry might gain Savitar's powers and start to struggle with the darker aspects that come with them. He could face challenges from his friends who no longer recognize him as the Barry they knew.
The key elements are the lightning strike and chemical exposure that gave him powers, his mother's murder, and his father's false accusation.