Well, it might involve Batgirl initially being a bit of an outsider among the Justice League. But then she proves her worth by taking on a powerful villain that the League has trouble with. For example, she could use her agility and stealth to infiltrate the villain's lair and gather crucial intel that the League needs to make their big move.
It could be the Joker. Since he's one of Batgirl's main foes in Gotham, his chaotic nature would pose a great threat to the Justice League as well when they enter Gotham's affairs.
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.
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.
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.
The Court of Owls would be a great main villain. They are a powerful and secretive organization in Gotham. Their connection to Gotham's history and their shadowy nature could pose a huge threat to both Young Justice and Batgirl. They might be after some meta - human power or trying to take over Gotham in a new way.
The Phantom Troupe could form an uneasy alliance with some of the more morally grey members of the Justice League, like Catwoman or Lobo. They might team up to take down a common enemy that the League can't deal with using their usual methods because of legal or moral constraints.
Brucie in this fanfiction could be someone who has been living on the fringes of society. He might be a street - smart person who has a deep understanding of the underbelly of the city. When the Justice League meets him, they realize that his knowledge can be useful in their fight against crime. He could also be a bit of an outcast, which makes his relationship with the super - heroes all the more interesting.
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.
Well, in such a fanfiction, there could be a lot of interesting scenarios. Maybe Batman would have to deal with the different leadership styles of the Justice League while trying to keep his family in line. The Batfamily members could show off their unique skills to the League members, like Robin's acrobatics or Batgirl's hacking skills.