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.
Captain America would probably be very reflective. He might see his past self as a reminder of his long journey and how much he has changed. He could try to offer some words of wisdom to his younger self.
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.
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One plot idea could be that the Avengers find Stan Lee's old unpublished comic scripts during an adventure.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.
Well, they might be shocked at first. Maybe their future selves have new powers or have faced some great losses that changed them. Their future selves could also give them warnings about upcoming threats that they'll face in the future.
One idea could be that the Avengers are accidentally transported to a movie set where the cast is filming their story. They initially think it's some kind of new villain's trick. Another could be that the cast members get to experience the powers of the Avengers for a day through a magical mishap.
Well, they might be shocked at how far they've come. Maybe their past selves would be in awe of their current achievements. For example, seeing the huge stadiums they now perform in compared to the small practice rooms in the past.