Well, Cisco Ramon from The Flash would be a great addition. He could use his tech skills to create some new gadgets or figure out ways to counter the villains' powers. Also, Martian Manhunter from Supergirl's side. His shape - shifting and telepathic abilities would add a whole new dimension to the story.
One idea could be that a powerful villain threatens both their cities simultaneously. Supergirl and The Flash have to team up, and in the fanfiction, their different powers and personalities could lead to some interesting interactions. For example, Supergirl's strength and The Flash's speed could be combined in unique ways to defeat the villain.
In this fanfic, the plot could be really interesting. Since it's a crossover, there could be a unique blend of the Supergirl and Flash universes. Kara's pregnancy might be a catalyst for a big change in the story. For example, it could lead to a new form of power or ability within Kara that is related to motherhood. Flash could be the one to first notice the changes in Kara's powers because of his quick thinking and observational skills. The story might also involve a race against time as they try to protect Kara and the unborn baby from a villain who wants to use the baby's potential powers for evil purposes.
Supergirl's strong sense of justice might sometimes make her a bit rigid in her views. Flash, being more adaptable, could find this a bit frustrating. However, when it comes to helping people, they both have the same goal. Supergirl's determination can drive Flash to be more focused when he's feeling a bit distracted, and Flash's ability to find humor in difficult situations can ease Supergirl's seriousness.
Maybe in the fanfic, there's a villain who can absorb their powers. So when Supergirl and Flash first encounter this villain, they start losing their powers bit by bit. This forces them to rely on their wits and non - superhuman skills to outsmart the villain and find a way to regain their powers before it's too late.
One idea could be that Flash and Supergirl team up to stop a powerful alien villain who is causing chaos across both their cities. The villain could have the ability to manipulate time and space, which would make it a real challenge for the two heroes.
In a fanfiction scenario where Barry is stabbed during a Flash - Supergirl crossover, it could lead to a lot of interesting plot developments. Maybe Supergirl would have to step up and use her powers in new ways to protect Central City in Barry's absence. She might have to team up with the other members of Team Flash more closely.
A great moment could be when they first meet and realize how different yet similar their worlds are. Supergirl might be in awe of The Flash's speed, and The Flash could be impressed by Supergirl's strength. It's a moment of mutual respect that often sets the tone for the rest of the story.
The characters would have a range of reactions. Barry (Flash) would probably be very protective. He's a hero who helps people, and Kara is his friend. He'd likely offer his speed and any resources he has to keep her safe. Kara's friends on Supergirl's side, like Alex, would be shocked at first but then become very supportive. They might start planning how to protect Kara and the baby from all the threats they face in their world.
First, you need to understand the characters well. Flash is known for his speed and his light - hearted personality, while Supergirl is strong, brave and has a sense of justice. Then, come up with an interesting plot. It could be a new villain they face together or a situation that challenges their abilities.