Comic books are littered with crossover events that bring heroes from across the universe together. These are often the most popular titles for Marvel because fans can read about a lot, if not all, of their favourite characters in the same comic.
This translates to the movies in the same way, in that one of the best aspects of films like Avengers Assemble, Age of Ultron and Civil War is that all the heroes come together in the same environment. Characters that may not have ever seen, interacted with or even known about the other characters.
The MCU so far has masterfully created a universe vast enough to house dozens of characters, yet have many of them never cross paths with each other. Until 2018's Infinity War, that is, which promises to be an event movie like none before it.
With Infinity War boasting appearances by almost all of the main characters introduced over the last ten years, plus a few new characters, there will be plenty of character crossovers and lots of new dynamics.
And while a lot of these will be first meetings between the characters, some of the most interesting will be between groups who are already familiar with each other...