Well, it could be that Dr Strange's use of the Time Stone at the last moment before the snap actually splits his existence. Part of him is snapped away, but another part remains. The remaining Dr Strange has to find a way to reunite his two existences and in the process, discovers a way to reverse the snap. He has to face challenges like his own fading power as a result of the split and the disbelief of the other survivors who think he might be a false version of himself.
In a 'Dr Strange survives the snap' fanfiction, perhaps he foresaw the snap in a different way than in the movie. He could have made a deal with a cosmic force. As a result, he survives but is burdened with the knowledge of what has happened. He starts gathering the remaining heroes secretly, using his magic to avoid detection by the forces that caused the snap. He trains them in new ways of combatting the threat and eventually they work together to bring everyone back.
One possible plot could be that Dr Strange uses his magic in a way that shields him from the snap's effects. Maybe he creates a temporal or dimensional bubble around himself just in time. He then has to figure out how to reverse the snap while dealing with the chaos that ensued. He might travel through different dimensions to find a solution or seek help from other powerful entities.
Thanos could still be the main antagonist. Just because Dr Strange survived doesn't mean Thanos is defeated. He might be on a quest to find out why Dr Strange survived and finish the job. Also, there could be other villains who emerge in the power vacuum left by the snap, and they could team up with Thanos or work against both Thanos and Dr Strange.
One possible plot could be that Peter has always felt a bit different, not just because of his spider - powers. Then he starts having strange dreams or visions that lead him to Dr Strange. Strange reveals that he is Peter's father. From there, Peter has to train in both his spider - skills and magical abilities. He might face a powerful enemy that only his combined powers can defeat. Along the way, he also has to deal with the reactions of his friends like Ned and MJ, who are shocked by this new revelation.
Obviously, Spider - Man (Peter) himself. And perhaps Aunt May, who would be very relieved that he survived but also worried about the state of the world. Ned could also be there, being Peter's best friend and all. They could be trying to figure out what to do together.
Suppose a new supervillain emerges who has the power to nullify technology and magic separately. This puts both Dr Strange and Tony Stark at a disadvantage. But with Peter Parker's quick thinking and unique skills, they find a way to combine their powers in an unexpected way to defeat the villain.
One plot idea could be that Harry discovers a hidden portal in Hogwarts that leads to Doctor Strange's Sanctum Sanctorum. He then has to learn the ways of the mystic arts to close a rift that is endangering both the wizarding world and the world of Doctor Strange. Along the way, he teams up with Doctor Strange and they exchange knowledge about their respective magics.
Maybe in the fanfiction, Peter survives the snap but his powers are affected in some strange way. He has to figure out how to control his new - or rather, altered - powers while also coping with a world that has been turned upside down. This could involve him teaming up with some unexpected allies.
One possible plot could be that Harry Potter, being the Master of Death, somehow finds his way into the Marvel universe. Dr. Strange, sensing this strange and powerful presence, seeks out Harry. They initially clash due to their different magical understandings. But then they realize they can learn from each other. Harry could teach Dr. Strange about the true nature of death and how to face it without fear, while Dr. Strange could show Harry more advanced spell - casting techniques in a different magical system.
She could survive by being away from the village during the massacre. When she returns, she is shocked. She decides to investigate the real cause of the massacre. Along the way, she discovers some dark secrets about her clan and the village. This knowledge gives her a new purpose, and she uses her Uchiha powers to bring the truth to light.
One possible plot could be that after surviving, Female Noble Six becomes a mentor. She uses her skills and experiences from the battles she's endured to train new recruits. Maybe she starts a secret mission to uncover the truth behind the events that nearly cost her her life, all while evading the enemy who still wants her dead.
One possible plot could be that Dr Strange is summoned to Hogwarts to train the students in advanced magic. He starts with Harry Potter, teaching him how to better control his magic and use it in more creative ways, like using time - based spells.