MCU: 8 Theories About Captain Marvel's Role In Avengers 4
4. She's In The Quantum Realm
The post-credits scene of Ant-Man and the Wasp sees Scott Lang becoming trapped in the Quantum Realm, with Hank, Janet, and Hope being snapped out of existence and leaving him without a way out.
It's been speculated ever since Infinity War that the Quantum Realm would play a big part in the sequel, and this only further adds to the case for that, but there's a good chance it'll involve Captain Marvel too.
There have been rumours Marvel will include the Quantum Realm, and in this movie we hear Janet van Dyne warn of 'time vortexes' within in. So, with Carol Danvers in the 90s and the potential for Ant-Man to go through time, could the pair meet in her movie? Might she too be in the Quantum Realm? That would explain where she's been, have a way of directly bringing her to the other Avengers - alongside a hero they know in Ant-Man - and be a means if involving time travel too, while there's also the potential that Marvel, given her powers, is actually the one who rescues Scott from the Quantum Realm.