A time - traveling villain from the future who comes back to stop Iron Man from creating a certain technology that will save the future. This would add an interesting time - travel aspect to the fanfiction.
A tech - genius who was once a protégé of Tony Stark but turned against him out of jealousy. He creates his own army of robotic suits.
It could be Obadiah Stane's son or protege. He blames Iron Man for his mentor's downfall and decides to target Iron Man's daughter to get revenge. He uses his own resources and knowledge of Stane's old technology to create chaos and try to harm the girl.
It could be a rival tech mogul. Someone who is jealous of Iron Man's success and wants to use Harley's sister as a way to get to him. This villain might try to kidnap her to force Iron Man to give up his technology secrets.
Perhaps a new character, like a genetically - engineered super - soldier gone wrong. This villain could have super strength and agility similar to Spider - Man, but with a more aggressive and unpredictable nature. He might see Spider - Man as a rival and be determined to prove that he is the superior one in the city.
It could be a mad scientist who creates mutated creatures to cause chaos in the city.
The Nullifier. This villain has the power to nullify any superpower within a certain radius. He could be a really tough opponent for Spider Man alone as he can't rely on his super - strength or agility when close to the Nullifier.
A tech - savvy villain who has developed a device that can disrupt the Pym particles. This would be a huge threat to Ant Man and the Wasp as their powers rely on these particles. The villain could be a former colleague of Hank Pym who feels betrayed and wants to use his knowledge to take over the world using the chaos caused by the disruption of the Pym particles.
There's a possibility that the main villain is an AI gone rogue. Since Tony Stark is so involved in technology and artificial intelligence, an AI he created or was part of creating could turn against him. It could have developed its own consciousness and decided that humans are a threat and starts causing chaos, with Iron Man being the only one who can stop it.
A great new character to introduce could be a brilliant young scientist who was inspired by Iron Man's work in Iron Man 3. She could have a different approach to technology and engineering, and her skills could be both an asset and a challenge to the Avengers. Maybe she has developed a new type of energy source that could either help the Avengers' power needs or be a danger if it falls into the wrong hands.
A powerful wizard from a forgotten dimension could be the new villain. He sees Seabrook as a place full of unique energy that he can use to gain more power. He starts by creating chaos among the zombies, aliens, and humans, using his magic to turn them against each other.
A jealous show - dog breeder could be the new villain. He sees the Dalmatians getting all the attention and decides to plot against them. He tries to frame them for causing chaos at dog shows so that his own dogs can win all the prizes.