Maybe there's a plot twist where the technology or powers of the three heroes get mixed up. Arrow suddenly has Supergirl's heat vision for a short time, Flash has Arrow's archery skills, and Supergirl has the Flash's speed force connection. This forces them to adapt quickly and use these new abilities in creative ways to fight a villain who was specifically targeting their normal powers.
A plot twist could be that a character from one of the shows, who seems to be a minor or unassuming figure, is actually a powerful being from the future sent to manipulate the events of the crossover. This person has been subtly guiding the actions of the heroes, and when the truth is revealed, it completely changes the heroes' perception of the events they've been through and who they can trust.
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.
Another theme is the exploration of different universes. Sometimes the fanfictions will send the characters into alternate realities where they have to adapt and find their way back. This allows for a lot of creativity in terms of world - building and character development. Also, friendship is often a key theme. These heroes come from different shows but in the crossovers, they form strong bonds and support each other through tough situations.
Another interesting idea is a mind - control situation. A powerful telepath takes control of some of the heroes and villains in their respective cities. Flash, Arrow, and Supergirl have to find a way to break the mind - control. They start by finding the source of the telepath's power. Along the way, they discover that the telepath has a tragic past that led to this misuse of power. They try to reason with the telepath while also fighting off the controlled allies and villains.
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.
In the fanfiction, their teamwork could be shown as a bit chaotic at first. Each hero has their own way of doing things. Arrow is more stealthy and methodical, Flash is impulsive and fast - acting, and Supergirl is more of a straightforward hero. But when they face a really tough enemy, they realize they need to combine their approaches. So Arrow might come up with a plan that incorporates Flash's speed for a quick execution and Supergirl's strength to overpower the enemy's defenses. They would have to communicate constantly and adjust their teamwork as the situation changes.
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.
One great Flash and Arrow crossover fanfiction is 'The City's Heroes United'. It has a really engaging plot that combines the unique abilities and personalities of the characters from both shows. The author does a great job of making the interactions between Flash and Arrow's characters feel natural.
There's also 'Crossover Chaos'. In this fanfiction, the Flash, Arrow, and Supergirl find themselves in an alternate universe where they have to figure out how to get back home while dealing with a threat that is unique to that universe. It's full of action and interesting character interactions.
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.