A throwaway line from Thor: The Dark World may hold a major clue to Avengers 4, and the inclusion of the Quantum Realm.
Director Alan Taylor's 2013 sequel gets a pretty bad rap in the Marvel Cinematic Universe, but it nevertheless involves key elements to the future of the franchise, such as Loki taking the Asgardian throne and the introduction of the Reality Stone. It now looks as if The Dark World may have put the pieces in place for the defeat of Thanos in Avengers 4.
Posting on Reddit, fan leafinferno recalls a conversation from the film in which Natalie Portman's Jane Foster is being examined by the Asgardian physician Eir, and notes the use of a quantum field generator. The important bit relates to Eir referring to the device as a "Soul Forge." Foster then asks, “Does a Soul Forge transfer molecular energy from one place to another?”
Others have already noted that the Soul Forge gives off an orange glow similar to the light of the Soul Stone itself; this same visual was seen surrounding Scott Lang in the Quantum Realm when Thanos snapped half of reality out of existence. It all ties together that the orange glow in the Quantum Realm could literally represent the trapped souls of Thanos' victims, and seemingly adds further evidence to Ant-Man being crucial in returning the fallen heroes.
Paul Rudd's Scott Lang is trapped in the Quantum Realm following the mid-credits scene of Ant-Man and The Wasp, but it's known he'll have expanded role in Avengers 4. There are multiple theories about how the alternate dimension could come into play in the sequel, and the importance of the Quantum Realm has already been teased by Tom Holland and Michael Douglas.
Unfortunately, Asgard was destroyed in Thor: Ragnarok. That said, Thor remains alive following the events of Avengers: Infinity War, and could work with the brilliant minds of Bruce Banner and Tony Stark to harness the power of the Quantum Realm. Although this theory is a million miles away from the various ideas that the Quantum Realm will help the Avengers travel back in time to get the Infinity Stones before Thanos, it's entirely possible that both ideas come into play.
As will all these Quantum Realm/Soul Stone theories, audiences will have to wait until May 2019 before they'll understand what is really going on. Until then, it's back to rewatching the MCU to try and pick up any clues to how Thanos can be defeated.
The untitled sequel to Avengers: Infinity War will release on May 3, 2019. Directed by Joe and Anthony Russo, Marvel’s Avengers 4 stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Samuel L. Jackson, Cobie Smulders, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Tom Holland, and Anthony Mackie.