Imagine that Voldemort has somehow made a pact with one of Tony Stark's enemies in the Marvel universe. Harry, who has been tracking Voldemort's new moves, discovers this and realizes that Tony is in danger. He uses his invisibility cloak to get close to Tony undetected and then uses his magic to give Tony an edge in the fight, like enhancing the power of Iron Man's repulsors with a magic boost.
Well, one way could be through magic. Harry Potter has a whole arsenal of spells. He could use a protection spell to shield Tony Stark from an incoming attack. Maybe Iron Man's suit malfunctions during a battle, and Harry, being in the right place at the right time, uses 'Reparo' to fix it.
At the start, Tony could be a bit skeptical of Harry's abilities. But when Harry shows off a spell, like making a Patronus, Tony would be impressed. They would then start to work together. Harry could use his magic to help Tony get into places that are otherwise inaccessible, and Tony could provide Harry with some cool gadgets. For example, Tony could give Harry a device that helps him communicate with the wizarding world more easily. They would learn from each other and form an unlikely friendship.
Well, it could work in a magical way. Maybe Harry's magical abilities could be combined with Stark's technology. For example, Harry could use magic to enhance Stark's suits, like making them invisible without the need for technology. And perhaps they could fight together against villains who are a threat to both the wizarding world and the world of technology.
The story could start with Fem Harry Potter being sent to the Muggle world for her safety. There, she meets Tony Stark. He is intrigued by her strange abilities. Deciding to adopt her, he takes her under his wing. She then has to adapt to a life full of technology and Stark's flamboyant lifestyle. Meanwhile, Stark has to deal with the magical aspects that come with having a witch in his house, like strange potions brewing in the kitchen and owls delivering letters at odd hours.
Well, in a way, Harry could introduce Tony to the magic world. Tony, being a genius, would be fascinated by the spells and potions. He might try to use his technology to understand magic better. For example, he could create devices to detect magic fields. And Harry could learn about advanced technology from Tony, like new ways of communication that could be useful in the wizarding world.
The storyline would be a wild mix. Harry would likely grow up with a scientific and technological influence from Tony. This could lead to him inventing some magical - tech hybrids. His approach to the Triwizard Tournament might be more strategic in a tech - based way. And instead of just relying on magic, he could use Stark's resources to uncover secrets about his parents' death and Voldemort's plans. He might even have a suit like Iron Man's but with a magical twist.
One way it could happen is through the power of the Time - Turner. Harry could use it to go back in time and prevent the event that injured Tony Stark in the first place. Or he could take Tony to Hogwarts where the advanced magic and potions could be used to heal him. The magic potions like the Felix Felicis could be given to Tony to enhance his chances of a speedy recovery, and the healing spells of Madam Pomfrey could also be very effective.
Maybe Harry travels to the Marvel universe and tries to learn from Tony Stark. He could be interested in Stark's technology for some reason related to fighting Voldemort, like using it to detect Death Eaters.
One place to look for such fanfiction could be Archive of Our Own (AO3). Just search for 'fem harry potter saves tony stark' on their site and you'll likely find a variety of stories. However, I can't specifically recommend a particular one without knowing more about your preferences in terms of writing style, plot elements, etc.
In one possible plot, Stark could be kidnapped by a powerful magical - technological hybrid villain. Fem Harry Potter, with her knowledge of both magic and her quick - thinking skills, tracks him down. She has to navigate through the unfamiliar high - tech world of the Marvel universe while also using her magic discreetly. Once she finds Stark, she uses a combination of spells and her own wits to defeat the villain and save him. This could also lead to an interesting friendship or partnership between them, with Harry teaching Stark a bit about magic and Stark introducing Harry to new technology.
One possible storyline could be Tony Stark bringing his technological genius to Winterfell. He might start by upgrading their defenses. For example, he could create some sort of energy shield around the castle, similar to the one he had in Stark Tower. This would protect Winterfell from the White Walkers and other threats. He could also work on improving their weapons, making swords and arrows with some advanced alloys he developed. Maybe he'd even introduce some basic robotics to help with tasks like scouting or carrying supplies. Another aspect could be his interactions with the characters there. He'd probably get along well with Bran, as they both have a deep understanding of different forms of knowledge. Stark could share his knowledge of the outside world with the people of Winterfell, and in return, learn about their ancient ways and magic.