Warning: The following contains SPOILERS for Marvel's What If...? episode 9.
The MCU finally assembled the Guardians of the Multiverse in Infinity Stone-powered Ultron who seems to rival the Watcher’s power, he has no choice but to step in to save the multiverse.
The Watcher’s unique Guardians of the Multiverse team brings together characters from nearly all of the previous What If…? episodes, finally tying the universes (and the show) together. The team, led by Doctor Strange Supreme, includes Erik Killmonger, Star-Lord T’Challa, Captain Peggy Carter, Party Thor, Black Widow (and Arnim Zola), and Gamora from an episode that was delayed to season 2 due to COVID.
Of course, not every Avenger could be part of the Guardians of the Multiverse team, though presumably there is a living version of each character somewhere in the MCU Multiverse who could have been chosen. However, there are a few Avengers known to be active in the show that the Watcher overlooks—namely Spider-Man, Hulk, Captain America, Captain Marvel, and Tony Stark. Where were these characters, and why were they not chosen for the team? The Guardians of the Multiverse are returned to their respective timelines at the exact moment they left, so any of these characters could have been chosen, but there are clues as to why the Watcher leaves some Avengers out.
Spider-Man
Spider-Man/Peter Parker from What If…? episode 5, entitled “What If… Zombies?!” is one of the only survivors of the zombie virus at the end of the episode. He is arguably the most important character because of his optimism in the face of all he’s lost. Along with an injured T’Challa and the head of Scott Lang, Peter travels to a hopefully zombie-free Wakanda in search of more survivors and a cure. The episode ends on an unresolved note, as the remaining heroes are en route to Wakanda where they will presumably have to contend with a Zombie Thanos who is in possession of five Infinity Stones. Things aren’t looking good since the roster of super-powered zombies continues to grow as the uninfected dwindle.
The zombie universe is one of two episodes that the Watcher does not pull a character from for the Guardians of the Multiverse team. In episode 9, Strange Supreme opens a portal and summons a horde of Zombies from that universe—including an incredibly dangerous Zombie Scarlet Witch—as fodder for Ultron, but that is the only way that What If... ? episode 5 is tied into the rest of the season. Leaving Spider-Man out of the Guardians of the Multiverse seems odd, especially because he seemed to be the main character of the episode. Perhaps the Watcher figured Peter had enough to contend with in his own universe... or maybe Zombie Thanos got them after all.
Hulk
Hulk/Bruce Banner, also from What If…? episode 5, “What If… Zombies?!” is last seen fighting a horde of zombies while his teammates escape Camp Lehigh in the Quinjet, headed for Wakanda. The episode takes place in an alternate universe right before the events of survive the zombie attack, but the Hulk could have. Hulk’s skin appears to be resistant to the infected bites of the zombies, and although the episode leaves it unclear whether or not he survives the horde, it is possible that Hulk could have eventually made it to Wakanda.
Assuming Hulk’s skin is impervious to the virus and that he escaped the horde, he seems like another missed opportunity for the Watcher’s Guardians of the Multiverse team. Much like Spider-Man from this universe, his addition would have tied the episode to the rest of the season in a more concrete way. However, there is the possibility that the Watcher left Hulk out because his universe was doomed in the end, and he ultimately became zombified as well. Furthermore, the Guardians of the Multiverse team didn’t necessarily need more unrestrained brawn since their strategy was more about distraction, finesse, and sleight of hand. Strange Supreme had the team covered in raw power, and Party Thor was an effective distraction to Ultron while other heroes tried to get close enough to him to snag an Infinity Stone off of his breastplate. The team didn’t need another heavy hitter, especially one who may not be able to control himself.
Captain America
The only known Captain America to be alive and have the super serum in the show is only revealed at the end of What If…">Captain Peggy Carter on its roster, so the super-soldier skill set was covered. There was really no need for a Captain America.
Captain Marvel
There were two other known and living Captain Marvels by the end of What If…? season 1. The most memorable version was the Captain Marvel who fought Party Thor in What If…">Ultron in What If…? episode 8, as she tries to stop his planet-destroying tirade. Granted, she had no team to back her up, but after her defeat the episode before, it probably would have been redundant to add Captain Marvel to the Guardians of the Multiverse team.
Tony Stark
Only one Tony Stark introduced in the season was alive for the final battle with Ultron in What If…">Hydra Stomper) and Gamora (who destroyed Thanos) are creating an Infinity Gauntlet. When the Watcher arrives, Tony is asking Gamora for help with his plan to “put a suit of armor around the world, around the galaxy,” and the Watcher dismisses him outright, taking only Gamora.
Though the Watcher's dismissal of Tony seems like another insult to the character, it’s clear why Tony isn’t chosen (and why the Watcher seems so mad at him). Some version of Tony created the Ultron that threatened the existence of the Multiverse. This Tony failed in restraining his egotistical creation since Ultron was able to take over Vision’s body, making him all the more powerful. Since the Watcher arrives to hear Stark repeat his flawed desire for a suit of armor around the world, a mantra that led to Ultron’s creation, it’s no wonder the Watcher doesn’t recruit Iron Man for the Guardians of the Multiverse team. As the creator of Ultron, Tony is essentially at fault for all of the destruction that the villain has wrought. The Watcher is rightfully disgusted with any version of Tony, especially one who could soon have access to an Infinity Gauntlet.
Any of these heroes could have been chosen by the Watcher for the Guardians of the Multiverse, regardless of where they were or if they were busy in their own universes. Each of the Guardians are returned to their universes at the exact moment they were taken, and some—like Captain Carter—were in the middle of missions when they were recruited. Ultimately, the Watcher knew which combination of heroes (and villains) he would need to assemble to defeat Ultron. Even though the Watcher was surprised by Ultron’s arrival and subsequent rise to power, he is still omniscient. He knew exactly which skill sets would be necessary not only to defeat Ultron but to trap the two problematic characters (Killmonger and Arnim Zola) in a pocket universe, watched over by Strange Supreme. The Guardians of the Multiverse both achieved their mission and brought Marvel’s What If…? season 1 to a satisfying and finally cohesive end, leaving audiences with a taste of what season 2 could be like now that the Watcher has broken his oath and could call upon his multiverse team again.