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.
Perhaps the Avengers are all affected by a strange magic that only Doctor Strange can reverse. As he tries to figure out what's wrong, he discovers that an old enemy of his has a vendetta against the Avengers and is using a spell that's slowly draining their powers. He has to race against time while also dealing with the Avengers' different personalities and powers in his Sanctum Sanctorum.
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.
One idea could be a battle where the Eternals and Avengers are initially at odds due to a misunderstanding. Maybe they both think the other group is causing some strange cosmic events. Eventually, they figure out it's a third party and team up.
One idea could be that Spiderman discovers a new threat in his neighborhood which turns out to be linked to a much larger plot involving the Avengers. So he has to convince the Avengers to take him seriously and work together to stop it.
One idea could be Deadpool trying to join the Avengers but constantly annoying them with his non - stop chatter and crazy antics. For example, he could keep interrupting their strategy meetings with his own hare - brained plans.
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.
A great plot could be that a member of the Avengers gets lost in space and is found by the Guardians. The Guardians then have to bring this Avenger back to Earth, but on the way, they encounter a series of challenges that require the combined skills of both groups. Along the way, they form strong bonds and exchange knowledge about their respective worlds.
One plot idea could be that the Avengers find Stan Lee's old unpublished comic scripts during an adventure. They then use the ideas in the scripts to defeat a new, powerful villain who is immune to their usual tactics. Another could be that Stan Lee, as a spirit, visits the Avengers to give them moral support and some secret wisdom before a big battle.
One idea could be that the Avengers need the Agents of Shield's help to track down a new, powerful and elusive villain who has technology that can disrupt the Avengers' powers. The Agents use their espionage skills to gather intel while the Avengers provide the muscle when it comes to confronting the bad guy.
A possible plot could be that the meeting causes some sort of time rift. The Avengers then have to fix this rift while also dealing with the differences between their present and past selves. Their past selves might be more naive or idealistic in some ways, which could lead to both comical and tense situations.
One plot idea could be that Naruto is transported to the Avengers' universe. He uses his ninja skills, like shadow clones, to confuse the villains. For example, when facing Loki, Naruto creates hundreds of clones, making it hard for Loki to target the real one. Another idea is that the Avengers learn chakra control from Naruto, enhancing their own powers.