Basically, all the people who loved the previous Spider-Man movies are coming back to watch this. There are people who liked those movies but may not care for the MCU as a whole plus your regular MCU fans. Then there is the crossover appeal of two characters as dissimilar as Strange and Spider-Man being in one movie plus the appeal of seeing all three movie Spider-men on screen together.
We might gripe about details but a movie like this actually happening and with a strong possibility of being descent and entertaining is like something out of a wild dream.
Strange has been more of a plot device since his first movie ended. His appearances in Thor Ragnarok, Infinity War, Endgame and No Way Home all seem to hinge on him making stuff happen for the sake of the story while Strange himself remained static as a character. That's got to get boring for someone like Cumberbatch after a while.
Nobody said it's not going to be horror tinged just that Strange himself seems to be getting lost in the mix in his own solo movie:
This movie is going to have to:
-follow up Wanda's plot about searching for her kids.
-introduced and get people to like America Chavez and unlike Hawkeye, this is a 2hr movie and not a 6 episode show.
-follow up Mordo's plot about wanting to kill all the magicians.
-introduce and set up a plausible threat like Shuma Gorath or Cthon or whatever they name the Lovecraftian horror they are going to fight.
-do right by fan favorite characters like Wong.
(I'll refrain from talking about any unconfirmed rumors about Professor X, Captain Carter or Wolverine showing up in the movie)
I should note that Winter Soldier and Ragnarok also had this many characters in their movies but managed to balance every one out. It helps that the titular character was the 'center' around which they built the story around. Ragnarok also had pretty extensive reshoots prior to it's release. Dr Strange 2 also had to deal with the pandemic which prevented them from shooting stuff they wanted hence the grueling six week reshoots. Hell, maybe Cumberbatch changes his mind after seeing the latest cut of the movie and we're all getting worried over nothing-that's also a possibility.
It's really depends on who is going to be moving the plot of the movie: Wanda searching for her kids? Mordo trying to murder all the magicians? Plucky new superhero America Chavez with her newly discovered super powers?