WARNING: The following article contains spoilers for Marvel's Ant-Man and The Wasp, in theaters now.
It's safe to say the superheroes of the Marvel Cinematic Universe will need a miracle come Avengers 4. Following Thanos' collection of all the Infinity Stones and his infamous snap that eliminated half of all life in the universe, an equalizer is a necessity if the remaining Avengers and Guardians of the Galaxy are expected to go another round.
The truth is, with or without his Infinity Gauntlet, the Mad Titan is quite formidable, so it seems it will require lot more brains than brawn to defeat him. For some time, however, there's been speculation about a potential trump card with the Quantum Realm, and how it could eventually factor into the right against Thanos.
As of Ant-Man and The Wasp, it definitely appears as if the microverse could indeed be the key in taking down the Mad Titan, for at least a few reasons.
The Time Vortexes
In the mid-credits scene, just before the Pym family disappears from existence, Janet (Michelle Pfeiffer) warns Scott (Paul Rudd) about coming across time vortexes in the subatomic void. They're places where space and time fold onto each other, blurring the lines of metaphysics and reality as we know it.
Once Scott learns how to work the microverse, as he would seem to be trapped there for the time being, hopefully he can exploit those time vortexes and head to the 1990s to enlist Captain Marvel to join the battle in Avengers 4; he can also pick up Avengers from the future and go hopping through the time stream, as set photos have hinted. Who knows, maybe he can even navigate the Quantum Realm as if it were his own Time Stone, creating an opening to stop Thanos before his snap.
Doctor Strange calculated all possible outcomes against Thanos in Infinity War by jumping ahead in time, so perhaps the one successful outcome he viewed is where he encounters Ant-Man and devises a plan (maybe that's the "endgame" he told Iron Man about) to counter Thanos and save the day.