Steve Rogers being the one seemingly killed on the train instead of Bucky Barnes, implying to viewers that he will become the Winter Soldier of this universe. Ultimately subverted, however, as the HYDRA Stomper armor survived the blast and Steve with it, albeit captured by HYDRA and passing the Tessaract into their hands.
The reveal that Thanos, the villain of the first three phases of the MCU in the Sacred Timeline, went through a Heel–Face Turn in this one, and is now a Ravager thanks to T'Challa talking him out of his genocidal ways and convincing him of other ways to prevent an Overpopulation Crisis in this universe.
The appearance of Ego at the end of the episode, in the restaurant where Peter Quill is working, indicating he'd moved his plan for the Expansion up by a few years and was ready to start.
What If... The World Lost Its Mightiest Heroes?
Tony dropping dead after Black Widow injects him with the lithium dioxide serum that was supposed to slow his palladium poisoning, as it did in the Sacred Timeline. In this universe, Natasha's attempt to help Tony was sabotaged to kill him instead... which serves as only the beginning of a murder spree that sees potential Avengers candidates being marked for death.
Bruce's Cruel and Unusual Death of literally being blown to pieces, revealed to be because a growing disk was attached to his heart causing it to increase in size rapidly.
The killer is revealed to be an insane Hank Pym, who lost Hope on a S.H.I.E.L.D. mission (implied to be Natasha's first encounter with the Winter Soldier) and swore revenge on Fury, leading to him murdering the Avengers.
What If... Doctor Strange Lost His Heart Instead of His Hands?
The Downer Ending — unlike every other episode where it's at least a Bittersweet Ending, this episode ends with Strange Supreme succeeding in obtaining enough power to save Christine from her inescapable death, but inadvertently leading to his entire universe being destroyed, including Christine. Strange begs Uatu to save everything, even offering to be punished for his actions, but the Watcher tells him straight-up that what happened is Stephen's fault and he cannot interfere with the consequences of his hubris. The immortal Strange is then left to exist alone in a tiny pocket dimension, surrounded by an endless black void with nothing but the immense guilt from his actions.
What If... Zombies?!
The Reveal that this timeline's version of Vision went mad and has been kidnapping people to feed the infected Wanda.
Another Downer Ending showing that Wakanda has fallen despite Okoye's claims that its shields protected it as the last sanctuary, followed by the reveal of an infected Thanos, five Infinity Stones at the ready, awaiting the arrival of the survivors with the last Stone in their possession.
What If... Killmonger Rescued Tony Stark?
For those who didn't see the trailer, Killmonger showing up out of nowhere and rescuing Tony from the bomb that would be instrumental in his capture by the Ten Rings, and his eventual transformation into Iron Man in the Sacred Timeline.
Killmonger murdering Rhodey and T'Challa and framing it as a mutual kill to incite conflict between the US and Wakanda.
Later on, Killmonger defeating the Mk II sentry prototype and using a vibranium spear to impale Tony, again used as a frameup to encourage retaliation against Wakanda.
What If... Thor Were an Only Child?
An alternate version of Ultron, in the body of the Vision and wielding all six Infinity Stones invading Party Thor's timeline in the final few seconds of the episode, a scene that even leaves Uatu shocked.
What If... Ultron Won?
This episode gives us the origins of Infinity Ultron, who is a far deadlier threat than his Sacred Timeline counterpart. After successfully uploading his CPU into the body that would become the Vision in the Sacred Timeline, Ultron kills most of the Avengers before wiping out nearly all of humanity (with only Hawkeye and Black Widow surviving) and murders Thanos who arrived with the other five Stones by cutting him in half vertically with the Mind Stone. He then uses the completed Infinity Stones to obliterate the likes of the Guardians of the Galaxy, Captain Marvel, and Ego in his genocidal rampage across the universe. After he completes his mission, rather than being left alone in a lifeless universe as he should have been, because the Stones gave him the power to see the Watcher outside his universe, this Ultron becomes aware of the Multiverse and sets out to take his genocide to the next level by ending life in every possible universe, even becoming strong enough to almost kill Uatu! As a result, Uatu has no choice but to go against his oath of non-interference if he wants to stand a chance against Infinity Ultron, and the episode ends with him requesting Strange Supreme's help.
What If... The Watcher Broke His Oath?
The opening montage of Uatu recruiting members of the Guardians of the Multiverse eventually comes to a universe where Gamora actually succeeded in killing Thanos, becoming known as "Gamora the Destroyer", and she's shown melting down the Infinity Gauntlet, and with a variant of Tony Stark wearing Sakaaran-style Iron-Man armor helping her!note The episode involving this universe had to be cut from Season 1 due to the COVID-19 Pandemic and was instead featured in Season 2
Despite being the Token Evil Teammate by a mile, Killmonger still manages to surprise everyone by taking control of the Infinity Stones after the Guardians defeat Infinity Ultron, only to end up in a Sealed Evil in a Duel situation with the digital consciousness of Arnim Zola inhabiting the vacant body of Infinity Ultron, both trapped for eternity fighting over the Infinity Stones and watched over by Strange Supreme in his pocket dimension.
The Stinger of the episode takes place in Captain Carter's universe, at the time of Captain America: The Winter Soldier during the infiltration mission on the ship, where Natasha shows her the intact HYDRA Stomper armor in a container, and even says that there's someone inside it (implied to be the Steve Rogers of her universe).
