BUT WHAT DID HAPPEN?
According to director Joss Whedon, during numerous rewrites of the script The Wasp was dropped from the ensemble and replaced with Black Widow. The reason for this seems to have stemmed from the fact that Black Widow - played by actress Scarlett Johansson - had already been introduced into the Marvel Movie Universe in Iron Man 2 (2010) and so it was easier to make use of an established character than try to introduce another new one.
As for Ant-Man, prior to the release of Avengers Assemble, a solo movie featuring the pint-sized superhero had been in the works, however it never quite got off the ground. Discussing Ant-Man's omission from Avengers Assemble, Marvel Studios' producer Kevin Feige suggested that Ant-Man would have been included if director Edgar Wright had made his movie as originally planned.
Feige said: "It's been eight years we've been working with him, and we finally decided we couldn't keep putting more people in. If Ant-Man had come out four years ago (2008), it would certainly have been part of the puzzle."
WAS THIS SUCH A BAD THING?
Although it would have been great to see both Ant-Man and The Wasp in Avengers Assemble (even if just in cameo roles) the movie was a box-office smash regardless, so it's not like they were really missed now was it?
BUT WHAT IF?
If Ant-Man and The Wasp had made it into the final script then jeez Louise, Joss Whedon would have had far too many characters to include in one film. As a result the movie would have had to run for approximately five days and civilisation would have ground to a halt. Yep, the end of civilisation just because of Ant-Man & The Wasp!