A plot could involve Supergirl being sent to Earth by mistake instead of Superman in a different timeline. She then meets the Young Justice team who are trying to figure out how she fits into their world. They could go on adventures together, like exploring an ancient Kryptonian outpost on Earth that holds secrets about both their powers and the future. Supergirl might also introduce them to some Kryptonian technology that gives the team an edge in future battles.
One plot idea could be that Supergirl, while on a mission in a new city, accidentally crosses paths with the Young Justice team. They initially mistake her for an enemy due to her Kryptonian powers being so strong. But as they get to know her, they realize she can be a great ally. Another idea is that there's a powerful villain who is specifically targeting Kryptonians and the Young Justice team has to work with Supergirl to stop this threat. They might learn new fighting techniques from each other during the process.
Perhaps there's a situation where a powerful alien race is threatening Earth. Supergirl, being from Krypton, has some knowledge about this race. The Young Justice team, on the other hand, has the experience of dealing with Earth - based threats. They combine their skills and knowledge. Supergirl could teach the team members with super - powers how to better control and expand their abilities, while the team helps her understand Earth's unique cultures and ways of fighting threats that are different from what she knew on Krypton.
Maybe Batgirl is sent by Batman to assist Young Justice on a mission. She has to adapt to their more light - hearted and less brooding approach compared to the Bat - family. Along the way, she forms a special bond with a particular member of Young Justice, perhaps Artemis. They have to overcome their differences, like Batgirl's strict no - killing rule and Young Justice's sometimes more lenient attitude towards villains, in order to succeed.
Another plot could involve Damian being sent on a mission by Batman to infiltrate Young Justice, thinking there might be a mole or some sort of internal problem. But as he spends time with the team, he starts to actually care about them and has to decide whether to complete his original mission or protect his new friends.
Maybe a plot where there's a competition between the two groups. A powerful and eccentric billionaire sets up a series of challenges across the city. The winner gets a large sum of money for their cause. At first, it's all about competition, but as the challenges get tougher and more dangerous, they start to cooperate and combine their resources to complete the tasks. This also leads to some friendly rivalries and new friendships forming.
Another plot could involve Raven being sent to the Young Justice world by some dark force. She has to adapt to this new place and team up with them to find a way back home while also dealing with the local villains. During this time, she could form unique bonds with the different members of the team. For instance, she might find a kindred spirit in Superboy due to their both being sort of outsiders in their own ways.
One idea could be that Nightwing has to train the Young Justice team in some new combat techniques. Maybe there's a new threat in Gotham that requires their combined skills to take on. Another could be a time - travel plot where the Young Justice team gets sent back in time and meets a younger Nightwing, and they have to work together to get back to their own time. Or perhaps Nightwing goes undercover in Young Justice to investigate a mole, leading to a lot of mystery and drama within the team.
One idea could be that the Bat family is investigating a strange series of crimes in Gotham. Meanwhile, the Young Justice team stumbles upon a lead that takes them to Gotham as well. They initially clash due to different methods of operation. Batman tries to keep his family in line while also dealing with the young, headstrong heroes of Young Justice.
One idea could be that Supergirl and Batman team up to stop a powerful alien weapon from being used on Earth. Maybe the weapon was stolen from a Kryptonian ship and Batman uses his detective skills to find clues while Supergirl uses her superpowers to fight off the bad guys who have the weapon.
One idea could be that they team up to face a new and powerful villain from another dimension. This villain has the ability to nullify their powers temporarily, so they have to rely on their wits and non - superhuman skills. Another plot could be that they discover a long - lost Kryptonian artifact that holds the key to saving Earth from a natural disaster, but they have to solve a series of complex puzzles to unlock its power. Or perhaps they meet in a situation where they initially don't recognize each other as Superboy and Supergirl, and there's some humorous misunderstandings before they figure it out.
In this fanfiction, The Flash and Supergirl could start off as rivals. They both think their way of protecting people is the best. But as they face a common enemy that is too strong for either of them alone, they learn to work together and respect each other's abilities.
In a fanfiction, it would be cool if Supergirl and Wonder Woman met during a time - traveling adventure. They could end up in different historical periods and have to work together to fix the timeline. For example, they might find themselves in the middle of a war in the past where a powerful weapon is about to be used that could change the future. Their combined powers of super - strength, flight, and Wonder Woman's combat skills and Supergirl's heat vision would be essential to stop the disaster. This kind of plot would allow for a lot of exploration of different time periods and cultures as well as their relationship.