There's also the possibility that the symbiote itself has become corrupted and turned against Spiderman. In this case, the symbiote would be an antagonist as it no longer wants to be with Spiderman and will fight to stay away from him while he tries to get it back.
It could be Venom. Since Venom is often associated with the symbiote in the Spiderman universe, he might be trying to keep the symbiote for himself and use its power for evil. So he would be a major antagonist in the story of Spiderman trying to get the symbiote back.
He might use his intelligence and knowledge of the symbiote's weaknesses. For example, if the symbiote has a weakness to high - pitched sounds, he could use some sort of sonic device to draw it out and then capture it. Or he could try to appeal to the part of the symbiote that once had a bond with him, by showing it that they were better together than apart.
Mephisto could also be an antagonist. He might be interested in Scarlet Witch's reality - warping abilities and try to corrupt her. Spider - Man would then have to try and save her from Mephisto's clutches while also dealing with the devil's minions. Mephisto could use his powers of illusion and manipulation to drive a wedge between them.
Doctor Octopus can also be a key antagonist in these stories. His intelligence and mechanical arms make him a formidable foe. He might create situations where Spiderman is forced to make difficult choices, and if those choices go wrong, it can send Spiderman into a downward spiral of self - doubt and darker actions.
Yes, there are quite a few. You can start by looking on fanfiction websites like Archive of Our Own. There are stories that explore the complex relationship between the Symbiote and Spiderman in very interesting ways, from the Symbiote's possessive love to the internal struggle within Spiderman about their bond.
Another possible antagonist could be Doctor Octopus. His brilliant mind and powerful mechanical arms would give Iron Man and Spider - Man a tough fight. He might be after some technology that Iron Man has or be trying to prove his superiority in the scientific and criminal world, which would bring him into direct conflict with our heroes.
It could be a new group of rogue ninjas who have a vendetta against Konoha. They might be led by a powerful and cunning ninja who has a personal grudge against Naruto or the village in general.
One common theme is the struggle for control. The Symbiote loves Spiderman but also wants to take over. Another is the idea of forbidden love, as their relationship is so complex and often dangerous.
Sure. Some fanfictions explore the idea in really interesting ways. They might play with the idea of the symbiote's possessive nature being a form of love. For example, in some stories, the symbiote protects Spider - Man in ways that are more than just a host - parasite relationship. It's really fascinating to see how different authors interpret this concept.
Most likely Dr. Doofenshmirtz. He always wants to be on top and if Phineas and Ferb get powers, he'll do anything to take them for himself. He might create some really wacky and dangerous inventions to try and steal their powers.
It could be a new super - villain created specifically for the fanfic. Maybe a scientist who has developed a formula that gives him the ability to manipulate energy and he wants to use it to take over the city. His powers would pose a threat to both Spider - Man and She - Hulk.