It could be a mad scientist. He could have been studying the Avengers from afar in their own universe and when they arrive in our world, he sees an opportunity. He might try to steal their powers or technology to create his own super - army.
Maybe a powerful sorcerer from our world who has been corrupted by dark magic. This sorcerer sees the Avengers as a threat to his new - found power and plans to defeat them one by one. He could use illusions and traps to make their lives difficult in this strange new world.
The Green Goblin could be a great main villain. He has a long - standing feud with Spiderman, and when the Avengers get involved, it could make the situation even more complex. His flying glider and pumpkin bombs would pose a serious threat to both Spiderman and the Avengers.
Well, it could be about the Avengers suddenly appearing in our modern world. Maybe they have to deal with things like modern technology that they're not used to, like trying to figure out how smartphones work while also fighting off villains who are using the chaos of their arrival to cause more trouble.
It could be Mysterio. He's a master of illusions. He might trick the Avengers into believing that Spiderman is a danger, using his illusions to make it seem like Spiderman is doing bad things. So the Avengers, thinking they're doing the right thing, kidnap Spiderman.
Thanos could also be a good option. Imagine Bella's planet holds one of the Infinity Stones that Thanos needs. He would stop at nothing to get it, and Bella and the Avengers would have to join forces to protect it. Thanos' brute strength and strategic mind would pose a huge challenge for them.
A powerful necromancer could be a great main villain. He tries to control the ghostly avengers for his own evil purposes, using dark magic to manipulate them.
It could be Loki. He's always causing trouble for the Avengers. He might see kidnapping a fan as a way to mess with Spider - Man and the Avengers, since he enjoys chaos and manipulation.
Obviously, Aladdin would be a main character. He could be a young man trying to find his place in the modern world. Then there's Jasmine. She might be a career - oriented woman, perhaps a businesswoman or a scientist. The genie would also be a key character. He could be a magical being trying to adapt to the modern world's rules. There could also be Jafar, who might be a corrupt politician or a powerful businessman out to stop Aladdin and Jasmine from achieving their goals.
Mysterio could be the villain. He's all about creating false realities. He could be tricking the Avengers into believing Spider - Man has gone rogue. Maybe he projects false images of Spider - Man attacking the city or stealing important items, which makes the Avengers go after him.
It could be Loki. He might have some new trick up his sleeve where he makes the Avengers forget Tony as part of a bigger scheme to cause chaos among the team and weaken their defenses. He could be planning to take over the world while the Avengers are distracted by this strange memory loss.
Maybe it could be a time - traveling criminal. This villain has the ability to jump between different timelines and has noticed that by disrupting the key events in both the Arrow and Avengers universes, he can create chaos and then rule over the resulting mess. He would constantly be one step ahead of the heroes as he could manipulate time to his advantage. He could also use his knowledge of future events to set traps for the Avengers and the Arrow team.