It could be Hydra. They might be trying to take advantage of the split between the Avengers and the Rogue Avengers. They see Peter as a wild card and try to either capture or turn him against both groups.
Another possibility is that the main antagonist is actually an AI or a technological threat. It could be something that was created as a result of previous Avengers' technology gone wrong. The Rogue Avengers and Peter have to figure out how to stop it, and they might have different ideas on how to approach the problem, which could lead to some interesting conflicts within their group.
There's a possibility that it's an old enemy of the Avengers who sees this as an opportunity. For example, Hydra could be involved. They might try to manipulate the situation to bring down both the rogue avengers and Peter Parker. They could spread false information to make the public turn against them all, while they carry out their own evil plans in the shadows.
Obviously, baby Peter would be one of the main characters. Among the rogue Avengers, it could be Captain America if he goes rogue in the story. Hawkeye might also be there as he has a family - like side and could be sympathetic to baby Peter. And Scarlet Witch could be involved too, with her powerful magic.
Probably Steve Rogers. He has that all - American hero charm and he values loyalty and courage, which Peter clearly has. Steve would recognize those qualities in Peter right away and would want to get to know him better. He might see Peter as a symbol of the future of heroism and be eager to befriend him.
In such a fanfiction, the Rogue Avengers might initially be suspicious of Peter. They could test his loyalty and skills. Maybe Peter has to prove himself worthy to join their cause against the establishment Avengers. For example, he could show off his spider - powers in a unique way that impresses them.
In this fanfic, the main antagonist could be a scientist who has been corrupted by his own experiments. He creates some sort of energy - based monsters that are immune to normal attacks. Captain Marvel and the Avengers have to figure out how to stop these creatures and the mad scientist behind them.
I think Thor could be really excited too. Thor loves a good warrior and Peter, with his spider - like abilities, would be an interesting addition to his view of heroes. Thor might be amused by Peter's small - scale heroics compared to his own grand battles and would enjoy seeing Peter's potential grow.
In this fanfiction, perhaps the rogue Avengers are hiding out when they hear baby Peter crying. They initially think it's a trap but then realize it's just an innocent baby. Some of the Avengers might want to take him with them because they see his potential, while others are more hesitant due to their own precarious situation.
In fanfiction, when the rogue avengers meet Peter, there could be a lot of conflict. The rogue avengers might be distrustful of Peter at first, seeing him as part of the establishment they are against. But then they could gradually realize he is just a kid trying to do good. And Peter might be in awe of them initially, but also confused about their actions. Eventually, they could team up for a common cause.
Well, in such a fanfiction, the Rogue Avengers might initially be suspicious of Peter Parker. They could mistake him for an enemy or a spy at first. But then, as they get to know him, they might see his potential and start to train him in their own unique ways. Maybe they'll share their different experiences from being on the run, which could be really eye - opening for Peter.
The main antagonist could be the Avengers' own bureaucracy or leadership structure. Peter might be yelling at them because their rules and decision - making processes are preventing them from doing the right thing. He might see this as a hindrance to their ability to protect the world effectively and is frustrated enough to yell at them to try and bring about change.