Warning: This Post Contains SPOILERS!
Most Marvel fans know how the Ant-Man and the Wasp post-credits scenes connect back to Avengers: Infinity War and possibly setup Avengers 4, but looking deeper into the recent revelations of Ant-Man and the Wasp, it's clear that the Quantum Realm could have a much more important role in the larger future of the MCU.
In Ant-Man and the Wasp, Hank Pym (Michael Douglas) and Hope van Dyne (Evangeline Lily) realize that Scott Lang's (Paul Rudd) venture into the Quantum Realm - and then back again - meant that Janet van Dyne (Michelle Pfieffer) could very likely still be alive down there. Over the course of the film, Hank, Hope, and Scott fight to get their expedition into the Quantum Realm off the ground, while forces like the desperate Ghost or the shady Sonny Burch try to stop them.
The climax of Ant-Man and the Wasp sees Hank venture into the Quantum Realm and save Janet, but Mrs. Pym comes back very much changed from her decades lost in the QR. Janet can now manipulate quantum energy in some kind of way, and uses it to help stabilize Ghost's quantum phase shifting. In the post-credits scene, Janet stages an expedition for Scott to retrieve more quantum energy inside the realm; unfortunately, the expedition takes place at the exact moment that Thanos erases half of all life in the universe - including Janet, Hank, and Hope. Scott is left stranded in the Quantum Realm, with no apparent way out.
The Ant-Man franchise has made a point to inform us that space and time work very differently in the Quantum Realm. Ant-Man and the Wasp takes things a bit further, by implying that The Quantum Realm could actually function like something of a junction point between all points in the space/time continuum. The biggest hint we get of this is during the post-credits scene, where Janet warns Scott that if he's not careful while in the Quantum Realm, he could be sucked into a time vortex that could deposit him who knows where in time (which Avengers 4 set photos suggest he indeed does). If the Quantum Realm is the crossroads of all physical space and time in all realities, then it can easily serve as the pathway between dimensions for all future MCU stories. We've already seen as much being teased in Doctor Strange, which clearly depicts the Microverse as one plane of existence that Stephen Strange gains knowledge of.
As stated, Janet van Dyne reveals that the Quantum Realm has "Time Vortexes," which can suck you up and spit you out, presumably in a random time period. That may be crucial for future MCU storylines, if time-traveling characters like Kang/Immortus come into play, or future storylines require time-hopping as a major hook. It's not just the Quantum Realm as a pathway through time, but also about a possible way of harnessing that temporal energy...
As Sonny Birch (Walton Goggins) explains to Hope van Dyne early in Ant-Man and the Wasp, The Quantum Realm isn't just some place people are hoping to explore and learn more about: it's the next arms race. Ant-Man and the Wasp reveals more of the potential power of quantum energy - and needless to say, the potential applications some wide-ranging. From clearly being able to power people like Janet with cosmic energy, to creating inadvertent monsters like Ghost, quantum energy is a definite game-changer for the MCU. And if the realm's temporal energy can also be harnessed, than threats to the timeline like Kang the Conqueror will definitely have a path to make their MCU debut.
Ant-Man and the Wasp's introduction of Laurence Fishburne's Bill Foster to the MCU, and the scene where we first meet him may reveal a lot about how the nature of the Quantum Realm may change the MCU forever. In the scene, Foster is giving a lecture to a room full of students, and the long-short of his heady science lecture is this: when a Quantum system is disrupted, the resulted chaos can create any number of parallel universes, and even if order is restored, the system will never be exactly the same. In other words: Thanos' infamous "Snap" may have disrupted the universe even more than the Mad Titan ever counted on, and when its restored, a new Marvel Cinematic Multiverse may have replaced the one we knew and loved.
The first Quantum Realm scene in Ant-Man had Marvel fans speculating like crazy that the cosmic entity know as Eternity can actually be spotted in the Quantum Realm. It's been a popular theory, and at the time of writing this, we're still sifting through all the possible Easter eggs show in Ant-Man and the Wasp's Quantum Realm scenes. If the theory holds any water, then the Quantum Realm will have some major connection to the most powerful beings in the MCU: maybe even their power source or home dimension.
One of the oddest and most sci-fi elements of Ant-Man and the Wasp is the notion of Quantum Entanglement. That heady scientific notion gets embodied by the phenomenon of Jane van Dyne's consciousness being melded with Scott Langs, after the pair made contact during Scott's first visit to the Microverse. The fact that two consciousnesses can become entangled in such a way opens doors of possibility for te kinds of mind-swapping, "ghost haunting," or intertwined fates (Kang/Immortus) that Marvel Comics have explored over the years.
Those are our theories about how the Quantum Realm could affect the future of the MCU in major ways - what are yours? Let us know in the comments!
Ant-Man and the Wasp is now in theaters. Avengers: Infinity War is also playing in theaters. Other upcoming Marvel Cinematic Universe movies include Captain Marvel on March 8, 2019, the fourth Avengers movie on May 3, 2019, Spider-Man: Far From Home on July 5, 2019, and Guardians of the Galaxy Vol. 3 in 2020.