If it's a crossover with Aladdin, Jafar could see Tinkerbell as a threat. He might want to use her magic to gain more power, so he tries to trick her into giving him her pixie dust or imprison her.
In a crossover with Sleeping Beauty, Maleficent could be the antagonist. Tinkerbell's presence and her magic of hope and light could be a problem for Maleficent's plans of darkness. So Maleficent would try to stop Tinkerbell from interfering with her curse on Aurora.
In a crossover with Snow White, the Evil Queen could be the main antagonist. She might be jealous of Tinkerbell's magic and try to capture her to use her pixie dust for her own dark purposes.
If it's a crossover with Alice in Wonderland, the Queen of Hearts could be the main antagonist. The Queen might see Tinkerbell as an intruder and try to behead her. Tinkerbell would have to use her wits and magic to survive in the crazy world ruled by the Queen of Hearts.
Madara Uchiha could be a great antagonist in this fanfic. Even though he was defeated in the original Naruto story, if he somehow gets resurrected and gets a chance to control the Juubi, he would be a formidable foe. He already had grand plans for the world in the original story, and with the Juubi's power, he could be even more dangerous. He might want to reshape the world according to his vision, and Naruto would have to fight him once again to protect everyone.
A powerful rogue Shinigami who has turned against Soul Society and is seeking to use Mugetsu Ichigo's power for his own evil purposes. This antagonist could have some dark secrets related to Ichigo's past or his family, which would add more depth to the story.
Dr. Doofenshmirtz from Phineas and Ferb would be a great antagonist. He could cross over into Jimmy's world and try to steal Jimmy's inventions to use in his own evil plans. His wacky schemes would clash with Jimmy's scientific logic, creating a lot of comical and action - filled situations.
It could be a power - hungry noble from the imperial court who sees Naruto as a threat to his own ambitions of ruling. This noble might have his own group of skilled fighters and use underhanded tactics to try and undermine Naruto's rule as emperor.
It could be Orochimaru from the Naruto universe. He might be interested in the cat's special abilities, perhaps thinking he can use them to gain more power or immortality. He would be a formidable antagonist as he has his snake - like powers and is very cunning.
In a crossover with the Pokémon universe, an evil Team Rocket - like organization could be the main antagonist. They might try to steal powerful Duel Monsters cards and use them for their own evil purposes. They could see the power of the cards as a way to take over both the Pokémon and Yugioh worlds. They might even try to combine the powers of Pokémon and Duel Monsters in some diabolical plan.
Dr. Octopus could be a great main antagonist. He could team up with a female super - villain from the Totally Spies universe. Together, they could create a plan to steal some top - secret technology. Dr. Octopus uses his mechanical arms to cause chaos in the city, while the female villain uses her own unique powers to keep the Totally Spies and Spider - Man at bay.
A powerful dark wizard could be the main antagonist. This wizard could be from a third, unknown dimension and is seeking to harness the combined powers of Ichigo and fem Oga for his own malevolent purposes. He might have the ability to manipulate dark magic and create illusions to deceive our heroes.
The Kingpin might be a good choice. He could see the Titans as a new weapon to use in his criminal empire. He could form an alliance with some of the more corrupt elements in the Attack on Titan world, like some of the military commanders who are only interested in power. He would use his wealth and influence to control the Titans and try to take over both universes. Spider - Man and the heroes from Attack on Titan would have to fight against his well - organized and well - funded forces.