WARNING: There are MAJOR SPOILERS for Ant-Man and the Wasp after this point.
Throughout much of Ant-Man and the Wasp, our plucky heroes have one major goal: rescue Janet van Dyne, the original Wasp, from the Quantum Realm. The Quantum Realm is a dimension of sub-atomic existence that bends the rules of space and time, and poor Janet has been trapped there for 30 years. It takes a few car chases, fistfights, and mumbo jumbo about "quantum entanglement," but by the time credits roll, Janet escapes and returns to our world.
However, by that time, she's absorbed so much energy from the Quantum Realm that she has become infused with a level of superhuman power. We only see her use this power twice: once to calm Hank's mind as the Quantum Realm's unstable nature makes him hallucinate, and again to stabilize villain Ghost's molecules so she stops uncontrollably phasing. There's also a moment earlier in the film where she speaks to her husband and daughter through Scott Lang, but this is attributed more to the nature of the Quantum Realm itself than Janet.
Because Janet only has a handful of minutes on-screen, we're not given time to dive into her new powers or indeed if she even still has them (see: questions we have after the Ant-Man and the Wasp ending). However, their origins could hint at the possibility of another Marvel superhero, one we've yet to see on-screen.
The hero that could be you
The first thing you need to know about Marvel comics canon is that it is infinitely weirder and more complex than the MCU. You think all the talk about magic space rocks for the past ten years has been a bit silly? Try the Enigma Force on for size.
In the comics, the Enigma Force is a sentient energy that permeates The Microverse, a parallel dimension that was created when a time travelling prince used a magic sword (I told you comics were weird!). The Enigma Force cannot act on its own - or perhaps chooses not to, it is said to be incomprehensible in its motivations and actions - and so instead will occasionally imbue heroes with power in times of great crisis. One way it does this is through the transference of the Uni-Power.
The Uni-Power, in the most simple terms, is a piece of the Enigma Force. In more poetic terms, it is a manifestation of every person in the universe's heroic potential. When the Uni-Power is held by a person (or alien, or dog, or what-have-you), that being becomes known as Captain Universe - a hero who has all the powers of... basically whatever the plot calls for at the time.
The Uni-Power has been bestowed upon many Marvel heroes, including Spider-Man, the Silver Surfer, Doctor Strange, and even Deadpool. It sticks around long enough to help avert disaster, and then returns to The Microverse where it re-merges with the Enigma Force until a new threat calls it into action.
A Microverse by any other name
Okay, that's all well and good, but what does it have to do with Janet? While the origins are different, the Microverse of Marvel comics and the Quantum Realm of the MCU could be mirror images of each other - much like how the Infinity Stones of the MCU are the Infinity Gems in the comics.
Ant-Man and the Wasp treats Quantum Realm energy as a sort of natural radiation; something that simply exists in that place. But what if the energy that allowed Janet to calm Hank's mind and heal Ghost is in fact the MCU version of the Enigma Force? Maybe it's sentient and maybe it's not, but either way it could give the Avengers an edge in their fight against Thanos in Avengers 4.
More immediately, it could help Scott escape from the dilly of a pickle he ends up in after the credits roll on Ant-Man and the Wasp. Marooned in the Quantum Realm with no one in our world available to pull him back out, Scott will likely have to find a way out of the Quantum Realm himself. If I'm being honest, I'd wager Avengers 4 will simply have Scott use his own wits to escape, possibly through one of those "time vortex" things mentioned in a moment of blink-and-you'll-miss-it foreshadowing.
But what if he doesn't?
Picture it: it's the climax of Avengers 4. What's left of The Avengers track down Thanos for a rematch, ready to... well, avenge their fallen friends. With the powerful Captain Marvel now on their side, we're sure that they can win. But they don't. Thanos continues to dominate our heroes, matching each punch they throw at him with a planet-rattling one of his own. The good guys are on the ropes.
And then Ant-Man hears a voice in his head, something alien and strange to him, but also familiar. It says it's here to help. The Uni-Power activates and empowers either himself or another member of the team, filling them with galactic power. Thanos gets punched in the face with the weight of a star behind a heroic fist. We all thought Captain Marvel would be our savior, but it turns out there are bigger, stranger things in the cosmos.
As the MCU continues to grow, to expand beyond our own planet and dimension, wouldn't it be cool if we got something as wonderfully weird as Captain Universe?