One great Supergirl crossover fanfiction is 'Supergirl and the Flash: Worlds Collide'. It has an exciting plot where Supergirl and the Flash team up to face a common enemy that can manipulate time and space. The story does a great job of capturing the essence of both characters and their unique powers.
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 best crossovers often explore the different power dynamics. Supergirl's super strength and flight compared to the Winchesters' hunting skills and knowledge of the occult make for an interesting mix. There are fanfictions where they exchange knowledge, like Supergirl learning about ancient spells from the Winchesters and the brothers getting tips on how to take down powerful foes from Supergirl.
You can start by searching on fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. Just type in 'Supergirl crossover' in the search bar and then look through the results. Sort them by popularity or rating to find the ones that are highly regarded.
Felicity Smoak from Arrow could also be a great focus. Her tech - savvy skills could be crucial in helping Supergirl deal with some high - tech threats she might face. Also, Winn Schott from Supergirl could have some interesting interactions with the Arrow team when it comes to building gadgets or hacking systems.
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.
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.
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 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.
Definitely, Cisco Ramon from Flash. He can add some tech - savvy humor and his ability to create new gadgets can be really useful in the crossover. For example, he could make a device that helps Supergirl deal with a new type of kryptonite - like threat.