Who could be the main antagonist in a 'Batgirl meets Young Justice' fanfiction?

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.

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.

It could be a new villain who has a vendetta against both Batman's family (including Batmom) and the Justice League. Maybe a powerful criminal mastermind who has studied both sides and found their weaknesses.

The Riddler could be a great antagonist. He might have a grand scheme that not only affects Gotham but also has implications for the Justice League's interests. His riddles could stump both Robin and the League members, forcing them to work together to decipher his plans.

Maybe a villain from the dimension where Wally has been. This villain could have followed Wally back to seek revenge or to take over the world that Wally has come back to. Since Wally has some knowledge about this villain, it would be his responsibility to stop the villain and protect his friends and the world. The villain might have powers that are a match for Wally's, making it a really tough fight.

A new, original character could be created as the antagonist. Maybe a powerful and vengeful assassin who has a grudge against Robin for some past event. This character could have unique abilities and a personal vendetta that drives the conflict in the fanfiction.

It could be a powerful sorcerer. This sorcerer might be trying to capture the Ghost Girl's essence for his own dark magic. He sees her as a source of great power because of her unique ghostly nature.

Maybe it's a new character created specifically for this fanfiction. A powerful alien scientist who sees Morty and young Rick as a threat to his plans for universal domination. He would constantly be trying to stop their adventures and capture them.

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.