Warning: SPOILERS for Ant-Man and the Wasp!
Ant-Man and the Wasp's shocking mid-credits scene hints at a major connection to Captain Marvel. As the follow-ups to the MCU-altering events of Avengers: Infinity War, Ant-Man and the Wasp and Captain Marvel are both prequels set before Thanos' successful quest to assemble the Infinity Stones. The mighty Captain Marvel doesn't appear and is not mentioned in Ant-Man and the Wasp, but the film's climactic moments could offer a major hint at the missing Carol Danvers. Of course, it involves the mysterious Quantum Realm.
At the end of Ant-Man and the Wasp, Scott Lang (Paul Rudd), Hope Van Dyne (Evangeline Lilly), and Hank Pym (Michael Douglas) succeeded in their mission to retrieve Janet Van Dyne (Michelle Pfeiffer) from the Quantum Realm. The original Wasp is about 30 years older than she was when she disappeared, and she has expectedly changed: she possesses Quantum healing powers, which she used to reset Ghost's (Hannah John-Kamen) molecules to keep her from permanently phasing out of existence. Janet remains a brilliant scientist and she later helped Hank built a miniaturized version of his Quantum Tunnel which fits in the back of a van. Their new mission in the end-credits scene is to send Scott into the Quantum Realm to retrieve Quantum healing energy to further help Ghost.
Here's where it gets interesting: prior to Scott entering the Tunnel, Janet gives him a few instructions: avoid the tardigrade field, which Hank ran into when he entered the Quantum Realm, and stay away from the "time vortex". Scott heeds her warnings and his mission is a success, but unbeknownst to Ant-Man, Janet, Hope, and Hank all fade to dust as a result of Thanos' finger snap. This leaves Scott trapped in the Quantum Realm with apparently no way to return (unless he brought a blue disc for his regulator, which is how he was able to escape the Realm the first time in the original Ant-Man). The film ends with Scott seemingly lost in the microverse, just like Janet was... and Captain Marvel possibly is.
It's been confirmed that Captain Marvel will revisit the Quantum Realm, though the manner of which isn't yet unknown. Here's where Janet name-dropping the time vortex could be hugely significant. Time travel will be a key component to next year's Avengers 4, but it may also explain where Captain Marvel has been for the past three decades. Could Carol Danvers' disappearance be tied to her somehow traveling through the time vortex? This would explain why Brie Larson's character doesn't appear to age over the past three decades. As for the device Nick Fury used to contact her before fading to dust in the end-credits scene of Avengers: Infinity War, if Carol is somewhere in the time vortex, Fury's ability to summon her could indicate that she is possibly there by choice and thus can exit at will.
We know from Janet's survival in the Quantum Realm for 30 years that it's possible, even without time travel. There are even hints of further life beyond tardigrades: just before Hank and Janet left the Quantum Realm, there is what appears to be a domed city in the corner of the screen. Everyone who has described the Quantum Realm has called it an endless void where nothing exists, yet it seems there is some form of life there, leaving it open for other characters to have inhabited it.
Captain Marvel should (hopefully) provide fans with all of the answers to her whereabouts between the 1990s and Avengers: Infinity War. As things stand now, Ant-Man is lost in the Quantum Realm and thus in the prime position to be the first to encounter Carol Danvers before they meet up with Earth's Mightiest Heroes in Avengers 4.