While she spent most of the film as the antagonist, by the end of Ant-Man and the Wasp the villain known as Ghost is on the side of the heroes, thanks to help (and some Quantum Realm energy) from Janet Van Dyne.
The movie ended with Ant-Man stuck in the Quantum Realm once again, as Thanos' snap from Avengers: Infinity War was finally affected across the universe.
However the trailer for Avengers: Endgame makes it clear that Scott Lang escapes. And this new theory speculates that it's all possible because of Ghost. Check it out:
Ghost joining the good guys on a permanent basis isn't out of the realm of possibility, and it does set up the possibility of the Thunderbolts in the Marvel Cinematic Universe. As actress Hannah John-Kamen said herself, Ava Starr is not a villain.
"I approach the character not like okay she's a villain she's a bad you know, she's a threat, definitely, to the protagonists," John-Kamen said to IGN. "She has an objective and she's the good guy in her mind and everyone else is bad. You've got to find, I find, the humanity in whatever you're playing when you are playing the antihero in a movie I think you really have to find the balance there of the threat but also the vulnerability with that, that comes with that."
"In this case, the story was always – spoiler! – to keep her around." Feige said to Birth.Movies.Death. "This was not the story of defeating a villain and everybody cheering. This is a different kind of villain story. When and where – in the tag, he mentions 'his new Ghost friend' – is something we’ll see. But there’s nothing better than struggling over the decision about what characters to bring into a movie, casting those characters, writing and putting them together in a way that works in the movie, and then have audiences go 'We love them, when are we going to see them again?' Shuri for sure is a great example of that. So with Ghost it is, step one: complete. As for step two, we’ll see where we go with that."
Avengers: Endgame premieres in theaters on April 26th.