An early draft of Joss Whedon's screenplay included The Wasp, and there are even pieces of concept art featuring the character alongside her fellow Avengers. Ant-Man was nowhere to be seen however, and The Wasp would later be removed. Though the filmmaker has since said that this early draft "was way too Wasp-y" it's more likely that Marvel asked that he remove the character in order to not interfere with Edgar Wright's plan for both her and Ant-Man. Wright constantly delaying that movie (before his departure) is why neither hero have joined the team yet.
