With Doctor Strange in The Multiverse of Madness hitting Disney+ in just a couple of weeks, fans can begin their deep dive into hints and easter eggs that will lead them into Doctor Strange 3. The introduction of Clea and her route into the dark dimension opens the doors to the potential return of characters from the first movie (such as Dormammu and Kaecilius).
Fans may also see some elements from Doctor Strange 2 carry over, such as America Chavez, different versions of heroes that fans know and love and even the escalation of the relationship between Clea and Stephen as in the comics. Fans will likely have to wait a couple of years to get any official announcement about a third instalment but the speculation can begin.
Clea
As a large number of the movie-going audience may not know, Clea is not only the niece to Dormammu, but she is the Sorcerer Supreme of the Dark Dimension and also ends up becoming Stephen Strange's wife. As fans have only seen her for around 10 seconds, they have no idea what her involvement in the second movie will be.
It's fair to assume that she will be a significantly main character in the sequel as she enlisted Doctor Strange's help. It wouldn't make sense for Marvel not to include her in a large proportion of Doctor Strange 3. As for whether she will become Doctor Strange's wife like in the comics, that remains to be seen.
The Dark Dimension
In the post-credit scene of Doctor Strange In The Multiverse of Madness, fans saw Charlize Theron appear as Clea as she opened a portal to the Dark Dimension that fans saw in the first movie. This obviously has numerous implications for other inhabitants of the Dark Dimension that could appear such as Kaecilius from the first movie as they were all trapped in the Dark Dimension in the third act.
It is currently completely up for speculation as to how much of the film will be set in the dark dimension and how much will be set in Earth-616 (the comics' Earth-199999), but it's likely that Strange will enlist the help of Wong yet again, so it's plausible that the film can take place partly on Earth and partly in the Dark Dimension.
Dormammu
With the inclusion of the Dark Dimension, comes the possibility of Dormammu returning. Marvel hasn't been one to reuse old villains for multiple movies, however, fans know that Dormammu is still alive. Strange only released him from the time loop if he promised to leave Earth and never return, but that didn't mention anything about Strange not travelling to the Dark Dimension.
Clea, played by Charlize Theron, is the niece of Dormammu so this also provides another point for the likeliness of his return. Marvel will only make Dormammu return if there is a logical, plot-centred reason for his involvement. So if his entry into the movie furthers the plot in some way, fans can potentially expect to see him return.
America Chavez
In the final act of the movie, America Chavez became an inhabitant of Kamar-Taj, learning to control her powers with Wong and the other sorcerers.
With the ability to jump between multiverses and being a great addition to Doctor Strange 2, it's incredibly likely that fans could see America Chavez help Doctor Strange and Clea as they try to prevent different universes from colliding in Doctor Strange 3. Having the ability to jump between universes would make preventing the collision a lot easier and America was a fan favourite character in the recent movie.
A Love Story
As fans saw in Doctor Strange 2, Christine Palmer from Earth-616 (the main MCU universe) has now moved on and married someone else. This opens the door for Stephen to carry on his love story with Clea as in the comics.
One of the great things about Doctor Strange 2 was that on the numerous occasions fans have seen Strange in the MCU, they have focused on him solely being a sorcerer and focused less on the human side of him (such as his feelings for Christine). After everything he has become and been through, he is still human and feels love like everyone else. Seeing more of his human side in Doctor Strange 3 certainly makes sense.
Incursions
When Clea appeared, she mentioned that Stephen's antics in Doctor Strange 2 has caused an incursion. An incursion is when the boundaries between two universes collide, erode and fall into each other. While Doctor Strange 2 introduced the concept of what an incursion is, it's likely that Doctor Strange 3 will explore this idea further.
As to how big the incursion will be, this remains to be seen and fans likely won't find out for a good few years until the third movie hits theatres. Marvel is crossing over its projects even more so it's likely that the incursion could have drastic effects on other movies and could even set the stage for a fifth Avengers movie.
Secret Wars
In the comics story of 'Secret Wars,' simply put, multiple universes collide until only two are left standing and an enormous battle ensues between the beings of different worlds. It's unclear whether Marvel will continue down this storyline but there are multiple arguments for the MCU going down this route aside from the fact that Incursions were the starting point that lead to the war.
Doctor Strange 2 already introduced the idea of the same characters from different universes. Marvel is also following the comic storylines far more than they did throughout the Infinity Saga. Finally, with the Fantastic 4 movie being released, fans could be introduced to Doctor Doom, who plays a key role in the Secret Wars story.
Stephen's Third Eye
In the final moments of Doctor Strange 2, it was revealed that Stephen from Earth-616 has a third eye similar to his What If...? and the evil Stephen Strange variant that fans saw in the third act of the movie.
This could tease that Stephen's is still cursed with the souls of the damned that he had to embody in the second movie. It's unlikely that Stephen will become evil in the sequel but the way that he embraces the eye in the post-credit scene could mean that fans see an even more powerful version of Doctor Strange than they saw in the second movie.
Kang The Conquerer
Kevin Feige has recently said that Kang and the events of Loki kicked off the events of Doctor Strange 2. The writers for the movie have also said that including Kang in the sequel could have overshadowed Wanda as the antagonist and might have "overstuffed" a movie that already has so many characters.
With Kang's effect on the multiverse carrying into Doctor Strange 2 and now the third film, without Scarlet Witch as the antagonist as she supposedly met her demise in the second movie, this certainly paves the way for the writers to include Kang as an antagonist for the third movie. While Kang will have his moment, it remains to be seen when and where that will be.
Christine Palmer
Fans saw multiple versions of Christine Palmer in the second movie and with Earth-616 Christine now married, it's likely that her character will not return for Doctor Strange 3 (unless there is a good reason for her to return).
That being said, even if Stephen continues his love story with Clea, Christine and Stephen are still very close friends and even though she is not as involved with multiversal beings as the version from her in 838, she will still always be around to help Stephen if he were to need it or be in danger. Whether Rachel McAdams returns for the third movie remains to be seen.