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.
A new tech - based villain could emerge. Maybe a brilliant but deranged scientist who is jealous of Iron Man's success and family. He sees Iron Man's daughter as a way to break Iron Man's spirit, so he devises elaborate plans to kidnap or harm her while also challenging Iron Man with his high - tech weapons and traps.
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 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.
Maybe the story could focus on Iron Man's daughter growing up in the shadow of her famous father. She struggles to find her own identity while also dealing with the expectations of the public. Eventually, she decides to use her intelligence in a different way than her father, like focusing on environmental technology to make up for some of the damage caused by Iron Man's battles.
A powerful tech - genius who has a grudge against the man of iron could be a great antagonist. Maybe this person was once rejected by the same company that made the man of iron's suit and now wants to prove that he is better.
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.
Obviously, Iron Man's daughter is the main character. She is often portrayed as a smart, strong - willed girl. Pepper Potts might also be involved as the mother figure or a mentor to her. Sometimes, Happy Hogan could be there as a sort of uncle figure, giving her advice or helping her out.
Perhaps a powerful alien race that has detected the unique energy signature of the Iron Man suit. They see it as a threat and send their best warriors to take it from Jack Darby. These aliens could have advanced technology and combat skills, making them a formidable foe for Jack. In the process, Jack might have to learn more about the suit's capabilities and even upgrade it to stand a chance against them.
Ra's al Ghul could be a great choice. He might see the daughter as a potential heir or a threat to his plans for Gotham. He could try to manipulate her or use her against Batman. This would force Batman to not only protect his daughter but also try to make her understand the true nature of Ra's al Ghul.
The Green Goblin could be a great main villain. He might target Peter Parker's daughter to get revenge on Spider - Man. Just like he did with Gwen Stacy in the original story, he could pose a serious threat to her.
Venom is also a strong candidate. In this fanfiction, he could be more powerful than ever. He might be infected with some sort of alien symbiote that gives him enhanced abilities. Venom could target Spider - Man because he wants to prove that he is the stronger one. And by attacking Spider - Man, he also draws the attention of the Avengers, which he sees as a challenge. He could start by attacking the city and luring Spider - Man and the Avengers into a trap.