One plot idea could be Doctor Strange traveling back in time to prevent a certain spell from being misused in the past that led to a dark future. He could face challenges like encountering his past self who doesn't believe him at first.
Another idea is that Doctor Strange travels through time to different key moments in the history of magic. He discovers secrets that change his understanding of magic and how it should be used in the present. For example, he might find out that a long - forgotten spell was actually the key to stopping an upcoming magical catastrophe. This would involve him having to piece together the remnants of this ancient magic and teach it to the present - day sorcerers.
Perhaps a fanfiction where Doctor Strange loses his powers and has to rely on his wits and the knowledge of ancient spells he has memorized to defeat a powerful enemy. This would show his growth not just as a sorcerer but as a person. For example, he could use his medical knowledge in combination with some basic magical principles to outsmart the foe.
One idea could be Doctor Strange accidentally sends the Avengers into an alternate dimension where magic doesn't exist and they have to find a way back using only their non - magical skills. Another could be that an ancient evil forces Doctor Strange to choose between sacrificing the Avengers or the entire world. Or perhaps a spell gone wrong makes the Avengers swap powers with each other, and Doctor Strange has to fix it while also dealing with an incoming threat.
Perhaps a time - travel based fanfiction. Christine accidentally gets sent back in time to a period when Doctor Strange was just starting to learn magic. She has to find a way to get back to her own time, and in the process, she ends up changing some events in Strange's past that have unforeseen consequences for their present relationship. Strange has to use his growing magical powers to fix things while also dealing with his feelings for Christine.
One idea could be that they team up to face a new, powerful interdimensional threat that neither can handle alone. Maybe a villain has found a way to merge different dimensions in a chaotic way, and Iron Man's tech and Doctor Strange's magic are needed to stop it.
One idea could be that you insert yourself as a new apprentice to Doctor Strange. You could start with little magical knowledge and learn from him, facing challenges like protecting the Sanctum Santorum from minor demons. Another could be that you are inserted into the story during the time when Strange is facing a powerful enemy and your unique modern - world knowledge helps him in an unexpected way, like using technology to enhance a spell.
One idea could be that Doctor Strange accidentally opens a portal to the wizarding world of Harry Potter. He could end up at Hogwarts and be mistaken for a new kind of wizard. Maybe he tries to use his magic to teach the students some new spells or ways of magic, but it causes chaos because his magic is so different from theirs. For example, his use of the Time Stone could mess with the time - turners at Hogwarts.
One idea could be that Lily travels forward in time and meets Harry as an adult. She gets to see all the good he has done and is filled with pride. Another could be Harry traveling back in time to save Lily from Voldemort, but in doing so, he risks changing the entire future. A third plot idea is that they both get trapped in a time loop together and have to figure out how to break it while also learning more about each other.
One idea could be that Doctor Strange discovers he is pregnant due to some strange magical experiment gone wrong. His powers start to fluctuate in unexpected ways as his body adjusts to the pregnancy. He has to deal with the reactions of his fellow sorcerers, some of whom are shocked and others who try to help him manage his new condition while still protecting the world from magical threats.
They could time - travel during a great war in the past. Kakashi and Sakura might find themselves in the middle of battles they only read about in history books. They could use their modern - day skills to change the outcome of certain battles. This would also have a huge impact on the present - day Naruto world as they know it.
One interesting plot idea could be that Harry travels back in time and meets Lily before she has him. He could try to prevent her death by warning her about Voldemort. Another could be that Lily time - travels to the future and sees the world after Voldemort's defeat, getting to know the man her son has become. Or perhaps they both travel to a different time altogether, like the time of the founders of Hogwarts, and have to adapt to that era while also dealing with their own relationship and the knowledge they bring from their own times.