Maybe a powerful witch who has been corrupted by dark magic. She wants to use the power of both Mystic Falls and Hope to open a portal to a dimension full of dark creatures. She's been secretly manipulating events in Mystic Falls for a long time and now that Hope has arrived, she sees an opportunity to carry out her evil plan. She can use her magic to create all sorts of illusions and traps to stop Hope from interfering.
A great plot for 'Mystic Falls meets Hope' fanfiction could be a time - travel scenario. Hope accidentally gets sent back in time to the early days of Mystic Falls. There, she has to navigate the complex relationships between the founding families and the emerging vampires. She could also learn new magic from the witches in Mystic Falls at that time, which she then brings back to her own time to use against a new enemy. And, of course, she might form unexpected friendships or rivalries with the people she meets in the past.
A powerful sorcerer who has been consumed by despair could be the main antagonist. This sorcerer might see the mage of hope as a threat to his plan of spreading darkness and hopelessness across the world.
It could be the Hidden Leaf Village itself. If the village turns against Naruto through false accusations or unjust treatment, it becomes the force that drives him to the dark side. The elders or some of the more conservative ninja in the village who oppose Naruto's unorthodox ways could be the face of this antagonism.
It could be an envious classmate. Let's say Hope is really good at academics and is always getting top grades. This classmate, out of jealousy, starts spreading rumors about Hope, trying to isolate her from her friends and undermine her confidence.
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.