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.
The Krypton OC could be from a different era of Krypton, one that had some secrets or experiments that are now lost. Barry Allen, with his speed, helps the OC explore Earth to find clues about their past. In the process, they fall in love. Their relationship is complicated by the fact that the OC is not fully human and Barry has to deal with the implications of being with someone so different, while also protecting the city from various threats.
Maybe the Krypton OC comes to Earth in a similar way as Superman, but lands in Central City instead. Barry Allen discovers the OC and at first is a bit wary. However, as they start to communicate, they realize they both have a strong sense of justice. They could team up to fight a common enemy that is using technology to disrupt both the OC's powers and Barry's ability to use the Speed Force.
It could start with Barry Allen noticing a strange energy signature in Central City. When he investigates, he finds the Krypton OC. The OC might be disoriented, having just arrived on Earth. Barry, being the hero he is, tries to help the OC understand where they are and what has happened.
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.
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 common theme is the discovery of new powers. Often, the OC will have some sort of power or ability that they are either unaware of or are just learning to control, and Barry helps them with that. Another theme is love against all odds. Their relationship usually faces some sort of obstacle, like Barry's duties as The Flash or some external threat.
One great Barry Allen OC fanfiction could be 'The Flash and the New Ally'. In this story, the original character might have some unique speed - related powers that complement Barry's. Another one could be 'Barry Allen and the Mysterious OC', where the OC has a connection to Barry's past that he doesn't know about. There are also stories like 'OC's Adventure with the Flash' which focus on the OC joining Barry in his battles and adventures in Central City.
Another theme is power exploration. Since Barry has his speed - based powers, the OC usually has some unique power too. The fanfics often explore how these powers interact, whether they complement each other or cause conflicts. For example, if the OC has a power related to time manipulation, it can create really interesting storylines when combined with Barry's speed. It can also lead to moral dilemmas about how to use their powers responsibly. And in some cases, the OC's power might be something completely new and different, like the ability to communicate with animals, which then leads to different kinds of adventures in Central City.
One common theme is redemption. The OC, who has been through abuse, often seeks redemption in a way, either by becoming a better person or finding a purpose in life with Barry's help. Another theme is trust. Since the OC has been abused, it's hard for them to trust, and Barry has to earn that trust throughout the story.
Well, an interesting plot twist could be that the OC's pregnancy is actually a way for her to gain new powers. As the baby grows, she starts to develop abilities that are linked to the baby's meta - human potential. This could put her in a difficult position as she has to learn to control these new powers while also dealing with the normal challenges of pregnancy. Additionally, there could be a twist where the baby is prophesied to be either the savior or the destroyer of Central City, and the OC has to figure out how to ensure the former while being pregnant.
To write a Barry Allen OC fanfiction, start with a strong concept for your OC. Are they a meta - human? A scientist? A normal person caught in extraordinary circumstances? Once you have that, plan the plot. Consider how your OC will meet Barry. Will it be during a battle? Or perhaps they work together at S.T.A.R. Labs. Next, develop their relationship. It could be based on mutual respect, or there could be initial distrust. Write in a way that shows how your OC grows and changes throughout the story, just like Barry does in the official storylines.
One idea could be that Barry Allen accidentally runs so fast that he breaches the multiverse and ends up in the Harry Potter world. He could use his super speed to help Harry and his friends in their battles, like quickly disarming Death Eaters. Another idea is that a spell from the wizarding world goes wrong and transports Harry Potter to Central City where he teams up with Barry to fight the Reverse - Flash. They could learn from each other's skills, Harry teaching Barry about magic and Barry teaching Harry about science and speed.