A powerful Decepticon could be the main antagonist. Maybe a Decepticon named Shockwave who is on a mission to find Bumblebee and extract some important information from him. He sees Dante as an annoying obstacle in his way and tries to eliminate him. Shockwave's advanced weaponry and cold, calculating nature would pose a great threat to Dante and Bumblebee's friendship and their goals.
A human villain could also work well. For example, a greedy industrialist who wants to capture Bumblebee and reverse - engineer his technology for his own profit. He hires a group of mercenaries to hunt down Bumblebee. This industrialist doesn't care about the potential harm it could cause and sees Dante as just another nuisance to be dealt with if he gets in the way.
There could be an ancient and powerful entity that has awakened in the world where Dante and Bumblebee are. This entity is drawn to the energy of Bumblebee and wants to consume it to become even more powerful. It has the ability to manipulate the environment and create illusions, making it a very difficult foe for Dante and Bumblebee to defeat.
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 new Decepticon who is trying to stop Charlie and Bumblebee from reuniting. This Decepticon might see their reunion as a threat to their evil plans.
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.
Maybe a powerful rogue ninja who has discovered a way to steal the powers of divine entities. He sets his sights on Amaterasu and Naruto because he knows that if he can combine their powers, he will be unstoppable. He creates various traps and illusions to separate them and weaken them so that he can carry out his evil plan.
A powerful new pirate group could be the main antagonist. They might be after the same treasure or territory as Luffy and Shanks. They could have some unique and dangerous abilities, like the ability to control sea monsters. Their leader could be a former crewmate of a well - known and feared pirate, who has his own vendetta against both Luffy and Shanks for some past reasons.
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.
Maybe a powerful warlord from Negan's world who somehow follows him into Artemis' realm. This warlord, with his own thirst for power and destruction, would oppose both Negan (if he starts to side with Artemis) and Artemis herself, wanting to take over the new world he has found.
It could be another god who is jealous of Zeus' interest in Atreus. Maybe a god who feels that Atreus is an outsider and doesn't belong in their divine affairs.