Who could be the main antagonist in a 'Dante meets Bumblebee' fanfiction?

One possible plot could be that Dante, perhaps a young and curious adventurer, stumbles upon Bumblebee in a hidden, abandoned junkyard. Dante is initially startled but quickly realizes Bumblebee is no ordinary machine. Bumblebee, being the friendly Autobot he is, tries to communicate with Dante through his radio sounds. Dante, with his quick thinking, starts to understand and they form an unlikely alliance to protect the junkyard from a group of thieves who want to steal valuable parts from the old cars there.

The Decepticons would be an obvious choice. They are the natural enemies of the Autobots and could pose a great threat to Dante as well.

Dante could initially be hostile towards Bumblebee, thinking he is a threat to his world. But Bumblebee's non - aggressive behavior might change Dante's mind. They could then start to exchange knowledge. Bumblebee could teach Dante about advanced alien technology, and Dante could teach Bumblebee about the magic and lore of his world. As they face challenges together, they would learn to trust each other more and more, and their interactions would become more friendly and collaborative.

It could be a powerful alchemist who has gone rogue and wants to use Naruto's chakra for some dark alchemy purpose.

There could be an ancient force that is awakened by the meeting of Harry and Hela. This force wants to destroy both of them because their combined powers could potentially stop it from achieving its goal of spreading chaos and destruction throughout all the realms. It would use all kinds of devious means to try and separate them and defeat them one by one.

Lex Luthor would be a great antagonist. He could see Superboy as a threat to his plans in Metropolis and try to find ways to discredit him or even eliminate him. Since Lois is a journalist, she might start to investigate Lex's actions against Superboy, which would put her in danger as well.

The World Government could be the main antagonist. They see both Luffy as a growing threat to their rule as a pirate and Dragon as a revolutionary. When Luffy and Dragon meet, the World Government might send their strongest agents like the Admirals to stop them from joining forces. The World Government's strict control over the world and their desire to maintain the status quo would put them in direct opposition to Luffy and Dragon.

It could be another powerful deity from either the Norse or the Christian pantheon who doesn't want Gabriel and Loki to team up. Maybe a more rigid and traditional angel who thinks Gabriel is consorting with the wrong crowd by interacting with Loki.

It could be an ancient and powerful vampire who has awakened from a long slumber. This vampire might see Alucard and Seras as a threat to his plans of ruling over all vampires.

It could be Orochimaru. He might want to use Ino's connection with the Kyuubi to his own advantage. Maybe he plans to extract the Kyuubi's power through Ino's mental link with it.