Well, the plot could involve Loki causing chaos in the Young Justice universe. Maybe he tries to manipulate the young heroes for his own gain, like getting access to some powerful artifact they are protecting. He could also form unexpected alliances with some of the anti - hero characters in the Young Justice world.
The more idealistic members of Young Justice like Robin might be immediately suspicious of Loki. He represents chaos and trickery, which goes against Robin's sense of order and justice. He would probably try to keep the team away from Loki until they know his true motives.
In such a fanfiction, there might be a big threat that requires both teams to combine their skills. The Flash could race with Kid Flash and share some speed - related tips. And Wonder Woman could train the female members of Young Justice in combat.
If Young Justice meets the Avengers in fanfiction, the cultural differences between the two groups could also be a factor. Young Justice is more of a teen - led group with its own set of values and goals. The Avengers are a more global - scale superhero team. Aqualad could share his knowledge of underwater threats with Namor if they ever interact. And there could be some misunderstandings at first, like when the Flash from Young Justice might accidentally run into Hawkeye's arrows during a fight against a common foe.
One possibility in a 'Young Justice meets Starfire' fanfiction is that Starfire gets involved in one of Young Justice's internal conflicts. For instance, if there are differences in opinion within the team about how to handle a certain situation, Starfire could bring a fresh perspective. Her unique upbringing on Tamaran and her experience with different cultures in space might help the team resolve their issues. Also, there could be a lot of character - development moments. The Young Justice characters could grow by learning from Starfire's positive attitude and her ability to see the good in people, even in the face of great evil.
The plot might center around a mystery that ties to both Young Justice's world and Batgirl's. For instance, some sort of ancient artifact that has the power to disrupt both the meta - human world and Gotham's criminal underworld. Young Justice brings their diverse powers and abilities, while Batgirl offers her detective skills and knowledge of Gotham. As they search for the artifact, they face various challenges and enemies.
The plot might involve Danny Phantom being mistaken for a meta - human in the Young Justice world. At first, there could be some distrust from the Young Justice members, but as they get to know him and his powers, they realize he can be a great asset. They could end up on a mission to stop a powerful ghost - like being that has invaded their dimension. Danny would use his knowledge of the ghost zone to help them understand this new threat and together they'd come up with a plan to defeat it.
One possible plot could be that Naruto is somehow transported to the Young Justice universe. He could team up with the young heroes. For example, Naruto's skills in combat and his ability to use chakra could be a great asset in their battles against super - villains. His positive attitude and never - give - up spirit might also inspire the Young Justice members.
The League of Shadows could be a great antagonist. They might see Young Justice as a threat to their plans in the world and Batgirl as an obstacle in Gotham. They could try to manipulate situations to make Young Justice and Batgirl turn against each other while they carry out their own devious plans, like releasing a dangerous virus in a major city.