Spoiler alert.
At the end of Avengers: Infinity War...
Seriously, spoiler alert.
...Thanos completed his goal of wiping out half of the universe's population including T'Challa, king of Wakanda and the current Black Panther. While many have speculated about this giving way to Shuri taking on the Black Panther mantle as she did in the comics, others are concocting theories on how T'Challa (and the rest of the possibly deceased) will make their way back to existence. (Because no one really believes Joe and Anthony Russo when they say they're dead for good, right?)
While Ant-Man and the Wasp is the next film in the Marvel Cinematic Universe, it takes place before the events of Infinity War. But from the looks of the trailers (and the fact Marvel hired Michelle Pfeiffer to play the O.G. Wasp), the film will likely circle back to the Quantum Realm aka the trippy subatomic level which works outside of time and space, where the original Wasp is trapped. Since the use of those same things—time and space—will definitely be needed to counter the effects of Thanos' epic snap, this could play a part in bringing the superheroes and the rest of the vaporized half of the universe.
On the other hand, Comic Book Resources writer Renaldo Matadeen dreamed up a theory about Black Panther's ancestral plane, referred to as the Djalia in Ta-Nehisi Coates' comic book run. In the film, it's seen as the place where the past Black Panthers reside in the afterlife. In Coates' Panther, it's where Shuri, who was killed by Thanos' minion Proxima Midnight, learns about Wakanda's history, culture, and mythology from a griot before T'Challa figures out how to create a rift into the dimension to bring her back. In both cases, this realm is a clear connection between the living and the dead.
Luckily for the MCU, it's the genius Shuri on the side of the living who could totally make a device to connect with the realm (since there aren't any more heart-shaped herbs for her to conveniently get there).
If this happens, and if T'Challa's there, she'll be able to make contact and get intel from him. If he's not there, it may prove our heroes' "deaths" may be more of a displacement of souls. This might mean Thanos trapped the "deceased" within the Soul Stone, a theory the directors have reportedly given credit.
Avengers 4 won't be out until May 3, 2019, so until then it's all speculation.
Ant Man and the Wasp will drop this July 6. Captain Marvel, who will certainly take a large role in Avengers 4 since Nick Fury buzzed her before he disappeared into the ether, will have her '90s-set origin story unfold on March 8, 2019.
