Warning: spoilers for Ant-Man and the Wasp to follow, as the plot may have revealed the villain for a hypothetical third entry into the sub-franchise.
A big mystery of the movie was the identity of its villain, the Ghost; the character is quite mysterious in the comics, and the film made things even more confusing by changing “him” into a woman, played by Hannah John-Kamen. Eventually we discover that she is named Ava and is the daughter Elihas Starr, who is portrayed by Michael Cerveris in a brief flashback.
In the comics, Elihas Starr is actually a significant Ant-Man villain, a genius mastermind who goes by the name Egghead. In the movie’s version of his backstory, he worked with the first Ant-Man, Hank Pym (Michael Douglas) but then betrayed him, getting fired—reasonably close to the source material, where he was found guilty of espionage. However, in the film, his use of stolen Pym technology created an explosion that apparently killed him and his wife, while also hitting Ava and giving her the ability to turn intangible.
Given that the Ghost was such a blank slate, one cannot help but think that those behind the film would not have chosen such an important character from the source material unless they had some sort of plan. Fans could thus easily theorize that Starr did not really perish in that explosion, and will return somehow as the villain of a future movie.
As for Ava herself, the movie reveals that her powers are unstable and risk killing her, until the heroes do what they can to try and help her. The post-credits scene even has Scott Lang/Ant-Man (Paul Rudd) refer to her as “our new Ghost friend,” and Marvel Studios co-president Kevin Feige has commented that was intended to be a very different kind of villain, and one who could return in future movies herself.
These lead to some interesting possibilities. Family has been a major theme of the Ant-Man sub-franchise—Ant-Man is the only MCU protagonist with a child, and Hope van Dyne/the Wasp (Evangeline Lilly) takes up her job from two superhero parents. Just as this movie had her trying to find her missing mother, Ava could discover her father is still alive in the next film and, with him fighting against her new allies, continue further on her interesting redemption arc.