One great Peter and Tony Endgame fanfiction is 'A New Beginning'. It focuses on how Peter copes with Tony's sacrifice in Endgame and his journey to become a better hero in his honor. Another is 'The Legacy of Iron Man and Spider - Man' which shows their bond even after Tony's death, with Peter carrying on his ideals.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions, and you can easily search for Peter and Tony Endgame - related ones. There are also some on FanFiction.net, though the selection might be a bit more mixed.
One way could be through some kind of time - related magic. Maybe Doctor Strange finds a way to manipulate time even more precisely than he did in the movie. He could create a special time loop that spares Tony's life while still achieving victory over Thanos. Another possibility is that Pepper, using her own intellect and some Stark technology she's been secretly working on, manages to reverse the effects of the Infinity Stones on Tony at the last moment.
A common plot involves Tony being resurrected by some kind of magic or advanced technology. For example, the Wakandans could have a secret method of bringing the dead back to life that they use on Tony. Or, in the Quantum Realm, there could be a phenomenon that reverses death, and Scott Lang discovers it and uses it to save Tony. Also, some fanfictions have Tony's soul being saved by a powerful being like the Sorcerer Supreme and then being put back into his body.
Peter could stand up to Tony. By showing his strength and not taking Tony's meanness lying down, he might earn Tony's respect. For instance, if Tony makes a snide comment, Peter could give a smart comeback that makes Tony see him in a new light.
One possible scenario could be that Peter sacrifices himself to save others during a crucial battle in the Endgame - like situation in the fanfiction. Maybe he uses his powers to hold off a powerful enemy while the rest of the heroes complete a vital task.
Maybe it's for a school project on modern pop - culture and fan - created works. They could be analyzing how fanfiction adds to the Endgame story and its impact on the fandom.
One aspect to consider in 'possessive Tony and Peter fanfiction' is the nature of their relationship. It might explore themes of protection. Tony could be possessive in a way that he wants to keep Peter safe from the various dangers in the Marvel universe, like powerful villains.
Yes, there are likely some fanfictions out there with this premise. Fanfiction often explores unique and unexpected relationships between characters. Writers might create a scenario where Peter Parker is scared of Tony Stark for various reasons, like Tony's power, his temper, or some miscommunication between them.