Maybe the Vulture. He's always on the lookout for new tech to sell on the black market. When he hears about Ant Man's unique shrinking abilities, he decides to capture Ant Man and force him to reveal his secrets. Spider Man steps in to protect Ant Man, and they have to fight the Vulture together.
Perhaps in the fanfic, Ant Man accidentally shrinks himself too much and gets lost in Spider Man's suit. Spider Man then has to figure out how to get him out without harming him. Meanwhile, they discover that there's a mini - world inside the suit with its own set of problems, like tiny villains causing chaos, and they have to deal with that together.
Spider-Man and Ant-Man have few direct connections in the comics. They might cross paths in team-ups or events, but their individual adventures are mostly separate. Spider-Man focuses on street-level crime in New York, while Ant-Man has more tech-based and sometimes cosmic adventures.
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.
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.
One possible villain could be Doctor Octopus. He might be back with some new and improved mechanical arms, and he wants to prove that he's the superior scientist and villain by defeating both Spider Man and Spider Girl.
Well, it could be about them teaming up to fight a new supervillain in the city. Maybe they start off not getting along because they have different fighting styles, but then they learn from each other and become great partners.
The Vulture could also be a major antagonist. His ability to fly and his sharp talons pose a great threat. Maybe he teams up with some other villains in the fanfiction to take on the two spiders at once. He could be stealing valuable items from around the city and Spider - Man and Spider - Gwen have to stop him.
The Green Goblin could be a great main antagonist even in a Spider Man reborn story. Maybe in this version, he has developed a new form of insanity - inducing gas that affects Spider Man's new powers. Another possibility is Doctor Octopus. He could have found a way to merge his mechanical arms with some sort of alien technology, making him far more powerful and a real threat to Spider Man reborn.
The Green Goblin could be a great main antagonist. He has a long - standing feud with Spider - Man. Maybe in this fanfiction, he could be out for a more brutal revenge after a particularly humiliating defeat. He could target Spider - Man's loved ones or try to completely destroy his reputation in the city.
It could be a character like the Hobgoblin. He's a classic Spider - Man villain and would bring a lot of chaos to the story.