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.
Martian Manhunter would be an excellent choice too. His shape - shifting and telepathic abilities can create some really interesting plot twists. He could help Flash and Supergirl by using his powers to gather information on their enemies or even impersonate them to gain an advantage in a fight.
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.
Jimmy Olsen could also be a great character to include. He is Superman's friend and colleague at the Daily Planet. His loyalty and his ability to get into some sticky situations can add a touch of humor and also some interesting sub - plots to the Supergirl and Superman fanfiction.
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 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.
Captain Cold could be an interesting addition. He might see the OC as a new target or an opportunity. Maybe the OC has something that he wants, like a valuable piece of technology. Or, he could form an unlikely alliance with the OC against a greater threat. Another character would be Killer Frost. If the OC has heat - based powers, they could have an epic battle or a complex relationship with Killer Frost. And of course, Harrison Wells, in any of his incarnations, could play a significant role. He might be mentoring the OC or using them for his own secret plans.
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.
Alfred should be included. He can provide Batman with support and advice, and his unique perspective on things could add an interesting element when Supergirl is around. He might even offer her some tea and wise words.
One great Flash and Supergirl fanfiction is 'The Team -Up of Legends'. It shows an alternate universe where they have to work together against a powerful new villain. The story does a great job of portraying their different powers and how they complement each other in battle.
Supergirl's strong moral compass can be put to the test. When she and the Flash encounter a situation where the line between right and wrong is blurred, like when dealing with a villain who has a sympathetic backstory. Her personality can grow as she decides how to handle the situation. The Flash's impulsive nature can also be developed. He might rush into things without thinking when working with Supergirl, and she has to reel him in. This interaction can show how they both grow and adapt to each other.
Sasuke would be a great addition. His complex relationship with Naruto could add an interesting dynamic. Also, Sakura could be included as Hinata's friend who gives her advice about her relationship with Naruto. Kakashi could also play a role, perhaps as a mentor guiding Naruto and Hinata in their new adventures. Shikamaru might be there too, using his intelligence to help them out in tricky situations.