Who could be the main antagonist when Buffy meets up with the wet works team in fanfiction?

Maybe they team up to fight a common enemy. Buffy's slayer skills combined with the wet works team's special abilities could make for a powerful alliance against vampires or other dark forces.

There might be an ancient evil entity that has been awakened. It starts to spread chaos and destruction, and Ruby Rose and Team SLVR are the only ones who can stop it. This entity could have some dark powers that are difficult to counter, making it a formidable antagonist.

It could be the Green Goblin. Since Harry has a connection to the Green Goblin in the original story, it would be interesting if the Green Goblin resurfaces in a new and more powerful form. Peter and Harry would have to put aside their differences and past traumas related to the Green Goblin to team up and defeat this common enemy.

A group of rogue Death Eaters could be the main antagonists. They think that Voldemort has gone soft by teaming up with Harry. So they break away and form their own more extreme and violent group. They start attacking both the wizarding world and anyone associated with Harry and Voldemort's new alliance. Harry and Voldemort must then fight against their former allies who have become a greater threat than ever.

Hawk Moth would be an obvious choice as he is the main antagonist in the original series. He could create an akuma specifically designed to target Ladybug when she is in a wet or water - related situation. Maybe an akuma that can control water currents and use them to attack Ladybug, making her 'wet for me' part of the akuma's strategy to weaken or distract her.

It could be a rogue ninja from a rival village. This ninja has a personal vendetta against Konoha and specifically targets Team 10 because of Naruto's presence. He might be jealous of Naruto's growing power and fame and wants to prove himself by defeating the team.

Perhaps an old enemy of one of the members of Team 9 shows up. Since Naruto has joined the team, they might see it as an opportunity to get revenge. This antagonist could be someone who was once defeated by the member's family and now wants to prove themselves. They would be very cunning and use their knowledge of the team member's past to try and divide the team.

It could be Danzo. He's always been a bit of a shadowy figure in the Naruto series. Maybe he sees Naruto as a threat to his power and influence in the village and manipulates the situation so that Team 7 has to abandon him. He would be a great main antagonist as he has the resources and the motives to cause such a rift in Team 7.

It could be Orochimaru. He might see the baby versions of Team 7 as perfect subjects for his experiments. He could try to kidnap them to use their unique chakra or potential for his own dark purposes.

It could be a powerful warlock who has found a way to manipulate time and space. He wants to stop Paige and Prue from joining forces because he fears their combined power. So he creates various obstacles and illusions to keep them apart.