Well, a cool plot could be that Doctor Strange discovers a prophecy that only the combined powers of the Avengers and his own magic can stop an all - powerful cosmic entity. So he has to gather the Avengers, some of whom are skeptical about his magic and the prophecy. Along the way, they face challenges that test their trust in each other. Another plot might be that a powerful magic artifact has been stolen and the Avengers suspect Doctor Strange at first, but then they realize it was a set - up by a common enemy and they have to work together to get the artifact back. And a third idea is that Doctor Strange's magic starts to malfunction and it's affecting the Avengers in strange ways, like making Thor's hammer too heavy to lift or Iron Man's suit go haywire. They need to find the source of the problem in the magical realm.
In a Doctor Strange and Avengers fanfiction, a great plot could involve time travel. Maybe Doctor Strange discovers that in the past, an event occurred that will lead to the destruction of the Avengers in the future. He decides to take the Avengers back in time to change this event. However, they soon find out that changing the past has unforeseen consequences in the present. For example, it could cause some of the Avengers to never become heroes. Also, they might encounter versions of themselves from the past who don't trust them. This creates a complex web of relationships and challenges that they must overcome to save their future selves and the world as they know it.
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 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 that the Avengers need Doctor Strange's help to enter an alternate dimension to retrieve a powerful artifact before the bad guys do. Strange uses his magic to create portals for the Avengers, but they face various challenges in the new dimension, like strange creatures and mind - bending illusions.
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 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.
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.
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 Sherlock's deductive skills are needed to solve a mystery in the magical world of Doctor Strange. Maybe a powerful artifact has been stolen and Sherlock's unique way of seeing details helps find it. Another could be that Doctor Strange uses his magic to transport Sherlock to different dimensions to solve a case that spans across realities. For example, a serial killer who can move between dimensions and Sherlock has to use Strange's magic to track him down.
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.
One idea could be that Doctor Strange somehow gets transported to the RWBY universe. Maybe he's there to help deal with a powerful Grimm that has magic - like abilities that the normal Huntsmen and Huntresses can't handle. He could use his magic to teach the RWBY characters new ways of fighting and also learn a thing or two from them about their unique combat styles.