What If...? is an animated Disney+ original series, set within the Marvel Cinematic Universe, based on the comic series of the same name. It is part of Phase Four and is the fourth Disney+ MCU series.
“Time... Space... Reality... It's more than a linear path... It's a prism of endless possibility... Where a single choice can branch out into infinite realities... Creating alternate worlds from the ones you know... I am the Watcher... I am your guide through these vast new realities... Follow me and ponder the question... What If...?”
What If...? flips the script on the MCU, reimagining famous events from the films in unexpected ways. Marvel Studios' first animated series focuses on different heroes from the MCU, featuring a voice cast that includes a host of stars who reprise their roles. The series is directed by Bryan Andrews; AC Bradley is head writer.
The Watcher examines the different universes containing divergences from their original paths. With each story, he finds a moral that connects to the victors and failings, all the while remaining anonymous and unseen by those inhabiting them. In one such timeline, he allows Stephen Strange to suffer with the destruction of his own universe and the death of the love of his life, despite him reaching out for help. In another timeline, he witnesses Ultron successfully defeat the Avengers and acquire the Infinity Stones; resulting in him becoming aware of the Watcher and wishing to break free from his universe so that he can destroy the others. With no one else to turn to, the Watcher enlists the aid of the Strange that he left in purgatory to help him.
The Watcher and Strange recruit Peggy Carter (from a timeline where she took the super soldier serum instead of Steve Rogers), T'Challa (from a timeline where Yondu took him into space), Gamora (from a timeline where she defeated Thanos), Erik Killmonger (from a timeline where he rescues and then betrays Tony Stark) and Thor (from a timeline where he did not have Loki as an adoptive brother). Together they become the Guardians of the Multiverse and make a plan to defeat the universe hopping Ultron. They return to to his timeline and recruit that version of Natasha Romanoff to fight Ultron with a stone destroyer, but learn that it only works in Gamora's universe.
Natasha reveals that she has Arnim Zola's consciousness on a hard drive and she and Peggy get it into Ultron resulting in Zola destroying Ultron from the inside. However, Killmonger attempts to take the stones for himself, only for a Zola controlled Ultron to fight him for it. Strange traps the two into an eternal struggle and promises the Watcher that he will watch over them. Everyone is returned to their universe proper, with the exception of Natasha, who is returned to a timeline where she died before the Avengers were formed. Peggy returns to her time, where she travelled to the future and discovers that Steve went into cryogenic sleep as well. The Watcher promises to watch over the Multiverse.
A.C. Bradley revealed that a pitch for a Guardians of the Galaxy-focused episode was rejected because it turned out to be half of the plot of Guardians of the Galaxy Vol. 3.
One rejected idea for the series was "What if Peter Parker turned into a real spider?", reminiscent of Man-Spider from the comics. This was rejected because it "was just too dark and too body horror for (a PG-13 rating)".
Originally, season one consisted of ten episodes. This was cut down to nine due to the COVID-19 pandemic, with the cut episode being pushed back to season two.
According to A.C. Bradley, the episode that was pushed back to season two was "more in the pure comedy vein", and would focus on Tony Stark. 
According to A.C. Bradley, while Season One focused on "big, let's end the world, let's kill everyone" arcs, Season Two episodes will "focus a lot more on the character stories and these heroes and showing a different side of them that people don't expect and hopefully they can relate to". 
According to Kevin Feige, "Season two will definitely incorporate movies from Phase 4" (Black Widow onwards). 
Gamora's episode was originally going to be in season 1, but was pushed back to season 2, due to COVID 19 complications and there was going to be a spin-off series, featuring on T'Challa/Star-Lord before Chadwick Boseman's death.