Doctor Strange in the Multiverse of Madness featured a number of fan-favorite characters from the Marvel Cinematic Universe, (and from outside the established MCU as well) as the multiverse begins to crumble around them, due in part to the meddling of both Doctor Strange and the Scarlet Witch.
While Multiverse of Madness features an abundance of characters both new and old, it also leaves many out of the action. Though several prominent MCU characters are not seen in the events of this film, there are several who are referenced, both directly and indirectly, despite their absence.
Tony Stark
Tony Stark's sacrifice continues to have profound effects on the Marvel Cinematic Universe years after his death. Early in Multiverse of Madness, Doctor Strange is confronted with his own decisions made during the Infinity War, which directly led to Stark's death at the end of the Battle of Earth.
In this film, Strange must grapple with the fact that his actions directly led to the death of one of the world's greatest heroes. Though he insists there was no other way, the loss of Tony Stark and the five-year gamble to defeat Thanos still weighs heavily on the former Sorcerer Supreme.
Donna Strange
During the confrontation between Doctor Strange and his dark doppelganger in the wreckage of a world caused by an incursion, it is revealed that Strange had once had a sister, Donna, who drowned in a frozen lake when her brother was unable to rescue her from the icy waters. Prior to this moment, there had been no indication of Strange having any siblings, let alone one who died as a child.
This moment of revelation brings an entirely new perspective to Stephen Strange, whose arrogance and cold demeanor went unexplained for years. Now, the character seems rewritten, as his veneer is revealed to have been delicately constructed by the mind of a traumatized child.
Chthon
In Marvel Comics, Chthon is the ancient demon that cursed Wanda Maximoff upon her birth on Wundagor Mountain. Multiverse of Madness directly references this supernatural entity when Wanda and Wong visit his temple on the mountain, finding a throne meant for the Scarlet Witch.
With the acknowledgment of Chthon, the Marvel Cinematic Universe seems increasingly willing to embrace the darker origins of Wanda Maximoff from the comic books. The introduction of this demon could also signify the coming of more devilish beings, such as Mephisto himself.
Hawkeye
When Strange visits Wanda to ask for her help with America Chavez, she asks him why he didn't look to other Avengers first. To this, Strange responds sarcastically regarding the Avengers, with a particular distaste for "the archer with the mohawk," of course in reference to Hawkeye, an original member of the superhero team.
One thing that MCU fans consistently love is references to other Avengers. Despite the cheeky reference, Hawkeye and Doctor Strange have never interacted onscreen in the MCU, though both remain active in the franchise, meaning that they could yet team up in the future.
Spider-Man
Doctor Strange in the Multiverse of Madness is a direct followup to the previous cinematic installment of the MCU, Spider-Man: No Way Home. While talking to America Chavez, Strange and Wong reference the events of the prior film, though they seem somewhat scarce on the details, despite having been there.
The short but memorable reference to Spider-Man is actually far more important than some audiences may realize. Strange's allusion to the hero confirms that he still remembers the events of No Way Home despite the memory wipe incurred at the end of the film. Given that Wong and Strange's memories seem hazy, this could indicate that the spell has merely rewritten memories, rather than altering the events of the past.
Dormammu
The film's mid-credits scene introduces Charlize Theron's Clea, a sorceress from the Dark Dimension who warns Strange of a coming incursion, indicating that the MCU is on a collision course with an as-yet-unknown alternate universe. Comics fans will know that this character has an intimate connection to the Big Bad from Strange's original film, Dormammu, Ruler of the Dark Dimension.
Clea's comic origin reveals that she is the niece of Dormammu himself, albeit far nobler than her uncle. While Dormammu himself has not appeared since being banished from Earth in the first Doctor Strange film, the arrival of his niece could signify his impending return.
Earth-616 Mordo
On Earth-838, America Chavez and Doctor Strange encounter a council known as the Illuminati, a member of which is a variant of Baron Karl Mordo. While the Mordo with whom Strange is familiar is not seen in this film, he is referenced as having tried to "ruin" Strange's life in the years since Doctor Strange.
Baron Mordo has been a lingering loose thread from the Infinity Saga, as his betrayal of the sorcerers has been left unexplored since 2016. However, the simple line delivered about Mordo attempting to ruin Strange's life at least indicates that the sorcerer has been active in the years since Doctor Strange, and that his former friend has been aware of his doings in that time.
Vision
In this film, Wanda Maximioff is depicted as being in the greatest state of grief in her entire life. She blames Strange for the death of her husband, Vision, whom she was forced to kill during the Infinity War, only for Thanos to resurrect and murder the synthezoid once more to obtain the Mind Stone, marking just one of many instances in which Wanda broke the audience's hearts.
Vision's death was a major turning point for Wanda, inspiring her to create the Hex over Westview in WandaVision. This death is yet another addition to Doctor Strange's growing guilty conscience, as Wanda views his gambling with the Time Stone as the primary reason Vision's initial sacrifice was undone.
Franklin And Valeria Richards
After breaking into the Illuminati chamber, Wanda Maximoff is confronted by Reed Richards of the Fantastic Four, who reveals to her that he has children of his own. Fans of the Fantastic Four will recognize this major reference to Franklin and Valeria Richards, the children of Reed and Susan Storm in the comics.
Franklin and Valeria Richards are incredibly important characters in Fantastic Four lore, who are both very powerful beings like their parents. While there had been no previous indications that the two Richards children were on their way to the MCU, this reference may indicate that they could be present in Mister Fantastic's next MCU appearance, whenever that may be.
Sue Storm
After being confronted by Mister Fantastic, Wanda Maximoff inquires about his children's mother, wondering if she is still alive, to which Reed replies that she is. This is, of course, a direct reference to Susan Storm, a highly-anticipated character likely to make her MCU debut in the next few years, as the franchise builds to the introduction of the Fantastic Four.
Sue Storm, a.k.a. The Invisible Woman, will be one of the next major comics superheroes to join the MCU, along with the rest of the Fantastic Four. While it seems unlikely that the Earth-838 Sue referenced in Multiverse of Madness will ever appear, the allusion to her character alone should be enough to keep Marvel fans going until her character makes her long-awaited first appearance in the franchise.