One idea could be that Supergirl's powers are misinterpreted by Shield as a new threat. So they send some of their agents to investigate. The agents then realize she's actually a hero and they team up to fight a common enemy like a powerful alien race that's planning to invade both their worlds.
First, you need to understand the characters well. For Supergirl, know her powers, her moral code, and her relationships. For Agents of Shield characters, like Coulson, Daisy, etc., study their personalities and abilities. Then, create a believable scenario for their meeting. It could be something like a shared threat or an accident that brings them together.
When writing an Agents of Shield crossover fanfiction, think about how the different technologies and magic systems (if applicable) will interact. If you're crossing over with a fantasy world, how will the Shield agents deal with magic? And how will the magical beings react to the high - tech gadgets? Also, focus on the character development. Let the characters from both universes grow and change as a result of their interactions. You can start with a small event that escalates into a big adventure for all of them.
One great Agents of Shield crossover fanfiction is the one where they cross over with the X - Men universe. It's really interesting to see how the characters from Agents of Shield interact with mutants. The story often has some great action scenes and character development as they face new threats together.
A mermaid in an Agents of Shield fanfic might also be a victim of some sort of Hydra experiment. They could have been trying to combine the unique physiology of a mermaid with their own technology or powers. This mermaid could then be on the run from Hydra and seek refuge with the Agents of Shield. The agents would then have to protect her while also trying to figure out how to use her abilities to their advantage.
The plot of such a fanfiction could revolve around a mystery that combines the two universes. There could be a villain who wants to steal Wonka's secret recipes and use them for evil purposes, and the Agents of Shield are the only ones who can stop it. They'd have to navigate the sweet yet dangerous world of Wonka's factory, facing off against the villain's minions while also trying not to get distracted by all the delicious treats. It would be a story full of action, humor, and a touch of magic.
One possible plot could be that Batman somehow gets transported to the world of Agents of Shield. Maybe a strange experiment by Hydra goes wrong and creates a portal. Batman would then use his detective skills to figure out what's going on with all the super - powered and high - tech stuff in the Agents of Shield world. He might team up with Coulson and his team to stop a common enemy, like a new super - villain created by a combination of Gotham's criminal minds and Hydra's evil plans.
One idea could be that the characters from Agents of Shield are sent to the location of Until Dawn. Fitz and Simmons could be analyzing the strange occurrences in the forest, similar to how they deal with alien and supernatural events in their own show. Meanwhile, the Until Dawn characters mistake them for part of the mystery and danger. The Shield agents would then have to use their skills and technology to survive and figure out what's really going on, while also trying to convince the Until Dawn group that they're there to help.
Well, one possibility is that the characters from Agents of Shield could bring their high - tech gadgets and strategic thinking into the world of Snake Eyes. For example, Fitz could create some new type of weapon or device that helps Snake Eyes in his missions. Another possibility is a clash of ideologies. The more morally grey areas that Agents of Shield sometimes delves into could contrast with Snake Eyes' more straightforward sense of honor. Also, a crossover could lead to some interesting team - up dynamics. Maybe Daisy and Snake Eyes could combine their combat skills in a fight against a common enemy.
One great piece of 'Supernatural Agents of Shield' crossover fanfiction is 'The Convergence'. It combines the spooky elements from 'Supernatural' with the high - tech and team - based action of 'Agents of Shield'. The characters from both universes interact in a really interesting way, like Dean Winchester being skeptical of all the high - tech gadgets at Shield.