For Sasuke, it could be a huge shock. Since they are rivals and friends, seeing Naruto do something so extreme would make Sasuke question his own role and his relationship with Naruto. He might become more determined to find out what led Naruto to this and seek revenge on those responsible. Sakura would be devastated. She has always cared for Naruto and seeing him harm himself in such a way would make her feel powerless and guilty. She might try to use her medical skills to fix the situation, even if it seems impossible.
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.
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.
Perhaps it's related to some sort of new power - up or transformation in the fanfiction. Sometimes in fan - made stories, the author likes to create unique and extreme situations for character development. Naruto might think that by removing his eye, he can unlock a hidden power within him. It could also be due to a misunderstanding. He might be tricked into believing that his eye is the source of some great evil that is about to befall the Hidden Leaf Village and so decides to rip it out to save everyone.
In this fanfiction, perhaps there's a big misunderstanding. Bucky has been led to believe that the arc reactor is some sort of key to a greater evil. So, without fully understanding its importance to Tony, he rips it out. This could then lead to a chain of events where Bucky has to face the consequences and try to make amends while Tony is fighting for his life.
It could be Bucky himself if he's under mind control. He'd be the one directly causing harm to Tony by ripping out the arc reactor, so in that sense, he's the antagonist, even if it's not really his fault.
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.
Morgan Stark, Tony's daughter, could be a central character. Her existence might be what drives the other characters to find a way to bring Tony back. Rhodey, as Tony's long - time friend, would surely be involved in any attempt to save him. And there could also be appearances from characters like Nick Fury, who has a knack for getting involved in important events and might offer some resources or knowledge to help Tony survive.