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.
One common theme is teamwork. Since the Flash and Arrow are from different shows, fanfictions often focus on how they can combine their unique skills to fight a common enemy.
Another theme is friendship. The fanfictions usually explore the developing friendships between the characters from the two shows. They might start off a bit distrustful but then grow to respect and rely on each other. A lot of stories show how Oliver and Barry become closer friends through their shared adventures.
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.
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.
Love and romance can be a theme in some cases, not necessarily between Arrow and Supergirl directly, but perhaps within their respective teams or with other characters they interact with in the fanfiction world. It adds an extra layer of complexity to the relationships and the overall story.
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.
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.
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.
In Arrow and Supergirl fanfics, the theme of friendship or romance can also be common. Some writers like to explore the possibility of a romantic relationship between them, while others focus on a strong platonic friendship. Additionally, the theme of facing a greater threat together is quite prevalent. This could be an alien invasion or a super - villain that is too powerful for either of them alone. Fanfics use these themes to create exciting and engaging stories.
Another theme is the combination of their powers. Many stories explore how the Arrow's combat skills and the Flash's super - speed can be used together to defeat enemies. It's like seeing two different forms of heroism merge into one strong force. This could involve them creating new strategies or tactics that are unique to their combined abilities.