One possible main antagonist could be a villain who is jealous of Tony's resources and sees Peter as a way to get to him. Maybe someone like Justin Hammer, who could try to manipulate Peter into betraying Tony and Steve.
It could be a new villain created specifically for this fanfiction. A super - powered individual who has a grudge against Steve for some past event and decides to use Peter as a pawn to get revenge on both Steve and Tony.
The plot might revolve around how Tony and Steve adjust their lives to fit Peter in. For instance, they have to learn how to be proper guardians. They could struggle with things like setting rules for Peter. On the other hand, Peter might introduce them to a more youthful and modern side of life. Maybe they all bond over building some high - tech gadget together in Tony's lab.
It could be a new villain who has the ability to manipulate minds. This villain could make Steve believe that he is a danger to Tony and Peter, causing him to leave. For instance, the villain might implant false memories or thoughts in Steve's head, making him think that his actions might accidentally harm them in the future.
There could also be other characters like Happy Hogan. He might play a role in helping Tony take care of Peter, like driving him to school or keeping an eye on him when Tony is busy. And perhaps Aunt May could still be in the picture, adding an interesting layer to the story as she and Tony interact regarding Peter's upbringing.
It could be Voldemort. Since he's the main villain in the Harry Potter series, he'd see Peter as another threat to his power, especially if Peter starts interfering in the wizarding world with his newfound family in Harry.
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.
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.
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.
It could be someone from Stark Industries who doesn't like the idea of Peter inheriting something. Maybe a rival within the company who sees Peter as an inexperienced kid and wants to take whatever Tony left for Peter for themselves.
At first, it's a bit rocky. Peter is used to being on his own, and Tony has his own way of doing things. But as they spend more time together, they find common ground. Tony helps Peter with his schoolwork, especially in science - related subjects. Peter, in turn, shows Tony the importance of having a pure heart in the superhero world. They go through challenges together, like fighting villains side by side, which deepens their bond.
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.