An interesting OC could be a villainous character in 'the flash oc fanfiction'. This OC could have the power to steal speed from others. They start targeting The Flash and his allies, causing chaos in Central City. The story could then be about The Flash trying to stop this new threat, while also trying to understand how this villain got their powers and if there's a way to reverse the speed - stealing ability.
Maybe the OC is a scientist who has developed a device that can temporarily steal The Flash's speed. At first, the OC uses it for personal gain, but then realizes the chaos it's causing in the city. They then team up with The Flash to take down other villains who are after the device. Or, the OC could be a meta - human with the ability to see the future in flashes. The Flash initially doesn't trust the OC, but as they start to have the same visions, they have to work together to prevent a major disaster from happening.
Maybe the OC is from a parallel universe where the rules of the Flash's powers are different. This could lead to a lot of exploration and conflict as Barry tries to understand and work with this new element in his world. The OC could bring new knowledge about speed force that Barry has never known before.
One idea could be that the OC has a unique power that complements Barry's speed. For example, the OC might have the ability to manipulate time on a small scale, and together they can prevent disasters that even Barry alone couldn't stop. Another plot could involve the OC being from a different dimension and Barry has to help the OC get back home while falling in love along the way. Or perhaps the OC is a scientist who discovers a new form of energy that can enhance Barry's speed even further, but there are some dangerous side - effects that they have to deal with.
One idea could be a crossover where a Twilight character gets super - speed like The Flash and has to adapt to a new world of crime - fighting. For example, Bella could suddenly gain the speed force and find herself in Central City, trying to figure out how to use her new power while also dealing with the local villains.
A Reverse Flash crossover with the Flashpoint timeline could also be great. The Reverse Flash could try to further manipulate the already - altered Flashpoint world. This could involve him trying to turn heroes into villains and vice versa, causing chaos in a world that is already on the brink of collapse.
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.
Maybe in the fanfiction, there is a device that can steal powers, and it targets both Danny and the Flash. They have to team up to stop the villain who has this device. Danny could use his ghost sense to detect the villain's presence while the Flash could quickly get them to the location. Their combined efforts would lead to a great adventure filled with action and some humorous moments as they learn to work together.
One idea could be a crossover of the Twilight universe and the Flash's world where their powers interact in unique ways during the kiss. Maybe the vampire's immortality and the Flash's speed combine to create a sort of time - frozen moment during the kiss.
A cool concept for twilight and flash crossover fanfiction could be a time - travel aspect. What if a character from The Flash travels back in time and accidentally alters the history of the Twilight world? This could lead to all sorts of chaos, like vampires ruling the world instead of just being in hiding. Or, a Twilight character could use the Flash's speed - related technology to gain an edge in their own battles, like using it to quickly escape from a werewolf pack.
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.