One potential main antagonist could be the Green Goblin. He has a long - standing feud with Peter Parker and could expand his enmity to include the Defenders. He might form an alliance with other villains to take on the combined might of Peter and the Defenders. Also, Mysterio could be a good choice. He could use his illusions to deceive the Defenders and Spider - Man, causing them to fight among themselves or lead them into dangerous traps. His skills in creating elaborate illusions would make him a formidable foe for the group.
Obviously, Peter Parker is one of the main characters. He's the friendly neighborhood Spider - Man. And then there are the Defenders. This group usually includes Daredevil, Jessica Jones, Luke Cage, and Iron Fist. So all of these characters would be the main ones in this fanfiction.
Perhaps a corrupted version of one of Peter's friends from another dimension within subspace could be the antagonist. This friend has been twisted by the strange energies of subspace and now wants to use Peter to gain access to the normal world to cause chaos.
The Green Goblin could be a great antagonist. If Peter's experiment gives him some new power or ability, the Green Goblin might see it as a threat and try to steal it for himself or simply destroy Peter to keep his own power in the city.
Perhaps in this fanfiction, Peter Parker meets the Defenders when they are all investigating the same strange occurrences in the city. There could be a mystery involving some sort of strange energy or new type of criminal organization. The Defenders with their unique skills and Peter with his spider - powers would start to piece together the clues. Along the way, they would have to overcome their differences in approach. For example, Peter is used to working alone most of the time, while the Defenders are a team. But in the end, they would combine their forces to solve the mystery and protect the city.
Deadpool himself could be considered the main antagonist in this case, as he is the one who is set on killing Peter Parker. His unpredictable and violent nature makes him a formidable foe.
A powerful dark wizard could be the main antagonist. This wizard might see Peter's newfound magical abilities as a threat to his own power and plan to either steal Peter's magic or destroy him. He could be someone who has been ruling the magical underworld in New York and doesn't want an upstart like Peter to disrupt his rule.
The Green Goblin could be a great main antagonist. Imagine if he also got his hands on some time - traveling technology. He could be causing chaos throughout different timelines, trying to establish his own rule in different versions of New York. Peter would have to chase him through time to stop his evil plans.
The Green Goblin could be a great antagonist even when Peter is just a kid. He could be starting out his evil deeds in a more low - key way that still catches Peter's attention. Maybe he's stealing from small businesses in Peter's neighborhood, and Peter starts to suspect something is up.
The main antagonist could be a scientist who wants to study the chibi. This scientist might be a bit of a madman, seeing the chibi as a unique specimen to experiment on and not caring about its well - being. He could try to capture the chibi and Peter has to stop him.
Mysterio could be another option. He could deceive Peter Parker five times using his illusions. One time he makes the city believe Peter is the bad guy, another time he creates false threats that Peter has to deal with, and so on. His ability to manipulate perception would make him a formidable and recurring foe.