Perhaps a dark force that exists in their world, something like an entity that feeds on negative emotions in art. This entity sees Bendy and Betsy's positive and creative energy as a threat. So, it tries to corrupt them and turn their beautiful art into something dark and menacing. It uses illusions and mind - games to try and break them down.
It could be a jealous artist named Max. Max has always been overshadowed by Bendy and Betsy's talent. So, he devises a plan to steal their ideas and take over the art world in their story. He starts spreading rumors about Bendy and Betsy to turn other artists against them.
A corrupted version of one of their own friends could be the main antagonist. For example, a possessed Boris from Bendy's world or a mind - controlled Mugman from Cuphead's world. This would add an emotional layer as they have to fight someone they care about.
One idea could be that Bendy and Betsy start off as rivals in an art competition within their strange world. They both have very different styles, Bendy with his more cartoonish, edgy style and Betsy with her elegant and refined art. As they compete, they gradually learn from each other and end up collaborating on a masterpiece that combines both of their unique talents.
The Devil could be the main antagonist. Since he is Cuphead's main enemy in his own game, he might follow Cuphead into Bendy's world. He could see Bendy's world as a new place to spread his evil and also wants to get revenge on Cuphead. He could start causing chaos in Bendy's world, which would force Cuphead and Bendy to team up against him.
Well, the two main characters are Bendy and Monika. Bendy, a character full of mystery and a touch of horror, brings his unique aesthetic from his dark, ink - based world. Monika, a digital entity with her own emotions and motives, adds a different kind of depth. In the fanfic, they are the ones who drive the story forward. Their interactions, whether it's a clash of their different natures or a coming - together in understanding, are what make up the heart of the story.
It could be an evil shinobi who wants to disrupt the meeting between Naruto and Kushina. Maybe they know that Kushina has some special knowledge or power that she could pass on to Naruto, and they don't want that to happen.
It could be a rogue Sound ninja who has his own agenda. Maybe he wants to start a war between Konoha and the Sound Village to gain power in the chaos. He would see Naruto as a threat to his plans and try to eliminate him.
Ra's al Ghul could also serve as the main antagonist. He might have a new plan for Gotham that involves using Catwoman as a pawn. He could be aware of her relationship with Batman and try to use it to his advantage. Batman would have to not only save Gotham but also rescue Catwoman from Ra's al Ghul's clutches.
A powerful and corrupt World Noble could be the main antagonist. They might be trying to stop both Luffy and Dragon's efforts for freedom. The World Noble could have a large military force under their control and be causing chaos and oppression in various parts of the world. This would force Luffy and Dragon to join forces to take them down.
Maybe a mysterious organization that is trying to capture all the Pokémon in the area where Ash and Misty meet again. They would be the antagonists as Ash and Misty would try to stop them. This organization might have some powerful Pokémon and devious plans that Ash and Misty have to figure out and defeat.
It could be a dark wizard who has infiltrated the Ministry of Magic. This antagonist might be trying to undermine Amelia Bones' work and also target Harry for his fame and power.