Maybe there's a time - traveling mishap that brings Flash and Supergirl to an alternate universe where their powers are reversed. Flash has to deal with super strength and heat vision, while Supergirl has to learn to control super speed. They must figure out how to get back to their own universes and in the process, they bond over their shared confusion and determination.
In a fanfiction, there could be an alien race that is immune to Supergirl's powers but is vulnerable to the speed force. Flash discovers this during a battle where Supergirl is struggling. He then uses his speed to create a barrier around the aliens, trapping them long enough for Supergirl to figure out a new way to defeat them, like using her super - breath in a new way that Flash helps her direct with his speed.
One idea could be that a powerful villain creates a time - space rift that pulls both Supergirl and Flash into an alternate dimension where they have to team up to find a way back home. Their powers work differently in this new dimension, and they have to learn to adapt quickly.
One idea could be that a powerful villain from another dimension is trying to take over all three of their cities simultaneously. The Supergirl, Flash and Arrow have to team up, each using their unique abilities. Supergirl could use her super strength to take on the villain head - on, Flash could use his speed to disrupt the villain's plans and steal important items, and Arrow could use his archery skills to set up traps and distractions.
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.
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.
One idea could be that a powerful villain from another dimension is causing chaos in both Supergirl's and the Flash's cities. They have to team up, with Supergirl using her strength and flight to combat the villain's minions on a large scale, while the Flash uses his speed to disrupt the villain's energy sources. Maybe they also discover a connection between their powers that helps them defeat the villain in the end.
In this fanfiction, The Flash and Supergirl could start off as rivals. They both think their way of protecting people is the best. But as they face a common enemy that is too strong for either of them alone, they learn to work together and respect each other's abilities.
One idea could be a crossover with another superhero show where The Flash has to team up with their heroes to stop a multiverse - threatening villain. Maybe he teams up with Arrow and they have to combine their skills to take on a speedster from another dimension who can also turn invisible.
Well, a great plot for Supergirl Arrow crossover fanfiction could be centered around a common enemy from another dimension. This enemy has technology that nullifies Supergirl's powers and Arrow's skills, so they have to rely on their wits and each other's unique strengths. For example, Supergirl could use her heat vision in a creative way to expose the enemy's weaknesses while Arrow uses his archery skills to set up distractions. They could also meet some interesting characters from the other dimension during their adventure.
One idea could be that an alien threat emerges which requires the skills of both Arrow's team in Star City and Supergirl in National City. They have to combine their combat styles, with Arrow's more grounded and stealthy approach and Supergirl's super - powered attacks. For example, Arrow could use his trick arrows to create distractions while Supergirl takes on the stronger alien foes directly.