A great storyline could be about a mystery that spans both their cities. They start investigating separately but soon realize they need each other's help. Team Arrow has the skills to infiltrate dangerous locations, while Team Flash can quickly travel between the two cities to gather information. There could also be a storyline where a powerful and ancient artifact is discovered, and both teams want to keep it safe from falling into the wrong hands. They would need to overcome their differences in approach to protect the artifact and the world from its potential misuse.
In fanfiction, when Team Flash meets Team Arrow, there could be an exchange of superpowers knowledge. The Flash's speedster abilities might amaze the members of Team Arrow, while the archery and combat skills of Team Arrow could be something new for Team Flash to learn from. Maybe they would team up to fight a common and extremely powerful villain that neither team could handle alone.
In a 'Team Flash meets Supergirl' fanfiction, there could be an epic team - up. The Flash might use his super speed to save Supergirl from a kryptonite - powered trap. Then they could join forces to fight a common enemy like Lex Luthor or an alien invasion. The scientific minds in Team Flash could also help Supergirl understand some new aspects of her powers.
Supergirl would be really interesting. Her alien origin and her super strength, along with her ability to fly, would make her stand out. She could bring a new dynamic to Team Flash with her different set of powers and her more idealistic view of heroism, which might contrast with some of the more street - level concerns of Team Flash.
They might find it really interesting. Some might be amused by the different interpretations of their characters in the fanfiction. For example, Cisco could be excited to see how the writer envisions his tech in new and creative ways.
Another possibility is that a character from Glee, say Rachel Berry, discovers she has some sort of meta - human ability similar to the Flash. The Flash has to come and train her to control it while also dealing with the chaos that her new powers cause within the glee club dynamic.
One possible plot could be a training session. Team 7, consisting of Naruto, Sasuke, Sakura and Kakashi, might meet Team Minato, which includes Minato, Obito, Rin and Kakashi in his younger days. They could engage in friendly sparring matches. Naruto would be excited to meet his father Minato and learn some new jutsu from him. The two teams could also exchange stories about their adventures, like Team 7 talking about their battles against the Akatsuki, and Team Minato sharing their experiences during the Third Great Ninja War.
In this fanfiction, there could be an epic battle of skills. Team Minato, with their unique abilities from their time, could test the mettle of Team 7. Maybe Minato's lightning - fast speed would be pitted against Naruto's new - found powers. It would be a clash of different generations of shinobi.
Barry could use his super - speed to quickly track down Zoom's lair. Once he locates it, he might try to create a distraction to draw Zoom away from Caitlin, allowing other members of Team Flash, like Cisco or Wells, to sneak in and free her. Cisco could use his powers to open up portals or disable any security measures that Zoom has set up.