The third and fourth Avengers movies that arrive in 2018 and 2019 are going to be the crossovers to end all crossovers. A decade of character development and world-building will result in a huge team-up to fight Thanos. But some further ramping up is required to prevent the films from seeming bloated.
The Avengers only had to combine a handful of characters that were introduced over 5 films (technically only 4 films since there has been a sequel to Iron Man), whereas these sequels will be the result of around 20 films and a huge amount of characters.
So we can probably expect a lot more crossovers in the next few years to make the combinations of heroes and allies seem more natural. It's already been hinted in the wake of Thor Ragnarok that we may get more "buddy up" movies, and it's a regular occurrence in the comics.
And there are some seriously good ideas for those cross-over events that Marvel should definitely heed. Some have been rumoured, some are mere speculation, but all would be awesome to see on screen...