Ant-Man and the Wasp: Quantumania has reportedly been given the working title of "GOAT Rodeo," according to social media reports. Fans have come to like hearing what these big comic book franchise films (particularly Marvel) choose for their working titles for films, as those working titles can sometimes hide clues and references to what the film is about. Then again, they can just as easily be in-jokes between the director, cast, and crew. In the case of Ant-Man 3, it could be either or both, given the implications of the (questionable) subtitle "Quantumania," and the deeper(?) meaning of the term "Goat Rodeo."
A "Goat Rodeo" is defined as being a "slang term for something going totally, unbelievably, disastrously wrong, and there’s nothing left to do but to sit back and watch the trainwreck." That's exactly the zany kind of comedic tone that director Peyton Reed and the Ant-Man franchise has chosen.
So far, we know Ant-Man and the Wasp: Quantumania is putting a few key characters and events into motion for Phase 4 of the Marvel Cinematic Universe. Actress Kathryn Newton is taking over the role of Scott Lang/Ant-Man's daughter, Cassie Lang, presumably as setup for the character's eventual evolution into a Young Avengers hero. In the comics she's taken on the superhero identities Stature, Ant-Girl, Ant-Man and Titan - so they have options when it comes to the MCU version (see also: Monica Rambeau).
The biggest thing has been introducing the next MCU big bad in Kang The Conqueror (Jonathan Majors), who is a dimension/timeline-hopping warlord. Since Avengers: Endgame established that the Quantum Realm is the highway through the multiverse, Ant-Man (Paul Rudd) and Wasp (Evangeline Lily) getting mixed up with Kang's armada is something that qualifies as "Quantumania." If Kang sets his sights on Earth and tries to use the Quantum Realm as a beachhead for his invasion, that would definitely qualify as a "GOAT Rodeo."
Here's Marvel's official announcement about the title and cast of Ant-Man and the Wasp: Quantumania:
Peyton Reed is back to direct the third Ant-Man film, Ant-Man and the Wasp: Quantumania. Paul Rudd, Evangeline Lilly, Michael Douglas, and Michelle Pfeiffer all return. Kathryn Newton joins the cast as Cassie Lang, and Jonathan Majors as Kang the Conqueror.