MCU Theory: Will Ant-Man Appear In Captain Marvel?
1. Avengers 4, Captain Marvel And Ant-Man's Freedom
Also, leaving Scott in there as a cliffhanger might as well be confirmation that it will be a major story point either before Avengers 4 or right at the start of it. And thanks to those Avengers 4 set leaks, we basically know that it has to happen BEFORE that film.
The set leaks confirmed not only that Ant-Man is involved in the "time travel" sequences, but also that he's wearing technology that seems to be the key to the time-travelling. And unlike all of the other heroes wearing the technology on their hands, his is built into his suit, suggesting he is the source of that tech.
We also know that Avengers 4 skips forward 5 years in time, so if Ant-Man doesn't escape, he's going to be in there a long time. Too long to be able to take existing technology built for one purpose in the Quantum Realm and repurposing it to do something else (using the time vortexes to time travel, presumably). In other words, Scott can't be trapped in the Quantum Realm that entire time.
So, he HAS to be out of there before Avengers 4 kicks off. And the ONLY way that can happen now is if Captain Marvel helps him in her own movie (or somehow begins to help him leading into another cliffhanger that will be resolved in the opening act of Avengers 4 before the 5 year jump.
There are other things to work out, like whether Captain Marvel is ever in 2018 or if Nick Fury pages her in the past, but for now it's enough to say that she MUST intervene to help Ant-Man escape.