It could be a powerful sorcerer who has turned to the dark side. This sorcerer might be using his magic to manipulate events in Sherlock's world for his own gain. He could be creating illusions and false trails to throw Sherlock off, while also using his magic to fight against Doctor Strange.
A time - traveling villain could be the main antagonist. This villain travels through time, stealing important artifacts from different eras. His actions are causing disruptions in the space - time continuum, which Doctor Strange can sense. Sherlock gets involved because the villain's actions are also causing strange events in his time period. The villain is trying to amass enough power from these artifacts to control all of time and space.
Maybe an evil genius who has somehow managed to combine magic and science in a dangerous way. He wants to use this combination to rule both the magical and non - magical worlds. He sees Doctor Strange as a threat to his magical plans and Sherlock as a threat to his scientific - based cover - ups. So he sets out to eliminate them both.
The Riddler could be a great main antagonist. He loves to create complex puzzles and mysteries, which would be right up Sherlock Holmes' alley. And of course, Batman has had his run - ins with the Riddler in Gotham. Holmes' deductive skills and Batman's combat and detective skills would be put to the test trying to outwit the Riddler's convoluted plans.
In this fanfiction, perhaps there is a mystery that spans different dimensions. Doctor Strange is familiar with the multiverse and different planes of existence, while Sherlock is an expert in solving mysteries in his own world. A series of disappearances start to occur, and the victims seem to be vanishing into thin air. Strange realizes that it has something to do with a portal between dimensions that has been misused. He teams up with Sherlock, who uses his sharp mind to figure out the pattern of the disappearances. Together, they close the portal and rescue the victims.
Neither might be fully dominant. They could end up being equal partners. Sherlock's deductive reasoning and the Doctor's knowledge of the cosmos could complement each other so well that they work side by side. They would respect each other's abilities and share the decision - making process. For instance, in a case where an alien is using human technology to cause chaos, Sherlock would analyze the human part of the situation while the Doctor figures out the alien technology involved, and they would jointly come up with a solution without one being clearly in charge over the other.
Maybe a rogue ninja who has somehow tapped into dark powers related to Kami's domain. This ninja wants to use these powers to take over the ninja world, and Naruto, with Kami's help, has to stop him. In this fanfiction, the rogue ninja could have some personal vendetta against Kami as well, which adds more depth to the story.
There's a possibility that an evil scientist from another planet could be the main antagonist. This scientist wants to use Bulma's knowledge and Cell's power for his own evil experiments. So he tries to break up their partnership and capture them both for his own purposes. He sends his minions after them and creates various obstacles for Cell and Bulma to overcome.
It might not be so straightforward. While Esdeath has the potential to be the antagonist, there could be other forces at play. For example, there could be a third party that is manipulating both Tatsumi and Esdeath for their own gain. In this case, both Tatsumi and Esdeath could be seen as victims of this greater evil, and they might have to team up against this new threat. However, if we consider the traditional view, Esdeath's actions and beliefs are in direct conflict with Tatsumi's, so she remains the likely antagonist.
Perhaps the main antagonist is an entity that represents chaos and destruction itself. It could be a force that is awakened by the meeting of Mewtwo and Arceus, seeing their combined power as a threat to its existence, and thus tries to stop them from achieving whatever goal they have in the fanfiction.
The main antagonist might be a being from another dimension who is able to manipulate both magic and the Hulk's gamma - irradiated energy. This being wants to use the chaos caused by the meeting of Harry and the Hulk to open a portal to let in more of his kind to take over all the universes. He sets traps for Harry and the Hulk and constantly tries to keep them from working together to stop him.
Perhaps an old enemy of Jiraiya who was thought to be defeated but has now resurfaced. This antagonist targets Boruto to get to Jiraiya, thinking that by harming Boruto, he can draw Jiraiya out and finally get his revenge. He would be a powerful and cunning foe, making it difficult for Jiraiya and Boruto to face him.
Perhaps a rogue group of mermaids and mermen. They don't like the idea of Triton allying with Percy, a half - blood. They believe that Triton should only focus on the pure - blooded sea creatures. So they become the main antagonists, constantly attacking Percy and trying to turn Triton against him.