Kang the Conqueror (Jonathan Majors), but Scott will also be ed by his now-teenage daughter Cassie (Kathryn Newton).
The original Janet Van Dyne was trapped there for decades before she was rescued in Ant-Man and the Wasp. Naturally, Ant-Man and the Wasp: Quantumania will further delve into the still-mysterious Quantum Realm, and it's probable that this is where Kang emerges from in order to threaten the world.
Despite the escalating, multidimensional stakes in this trilogy, the Ant-Man films are the most broadly comedic in the MCU, and this is now pointedly exemplified by Ant-Man 3's title leaning into how the franchise overuses the word 'quantum'. The third chapter may be titled 'Quantumania' but the dialogue in Ant-Man and the Wasp is literally its own version of 'quantum mania'. Just about every concept in the second Ant-Man movie is prefaced with the word 'quantum'. Between Hank, Hope, Janet, Ghost (Hannah John-Kamen), and Bill Foster (Laurence Fishburne), 'quantum' is said numerous times and for basically every occasion, to the point where Scott hilariously (but sincerely) quipped, "Do you guys just put the word 'quantum' in front of everything?"
Scott was the audience's surrogate because the absurdly funny barrage of words with quantum said in front of them in Ant-Man and the Wasp was inescapable: Quantum phasing. Quantum spectrometer. Quantum anomaly. Quantum research. Quantum entanglement between the quantum states. Quantum technology. Quantum energy. Quantum system. Quantum healing particles. Quantum tunnel. Quantum void. Of course, Marvel Studios was fully aware of how ridiculous the quantumania got in the sequel and they even mocked it in a special feature on Ant-Man and the Wasp's Blu-ray. Giving Ant-Man 3 the subtitle Quantumania may be an overt hint that the saga's quantum lunacy will reach its apex and the film will keep the quantum jokes coming.
Meanwhile, Kang is an Avengers-level villain, and with him as the film's big bad, the quantum stakes in Ant-Man 3 will no doubt be the highest they've ever been, and the extended Lang/Pym/Van Dyne family will find themselves in severe Quantum jeopardy. Further, now that's she's grown up, Quantumania will be Cassie Lang's superhero coming-out party if she gets her own size-changing powers and becomes Stature, and this will surely be the ultimate test of Scott's Quantum parenting skills. Of course, the Quantum Realm is central to Ant-Man's prominent and increasingly growing corner of the MCU and that will continue to be the case in Ant-Man and the Wasp: Quantumania. After all, it's literally in the title, which is itself a clever self-own.
