The Zombie Doctor Strange isn't the only Strange thing about Doctor Strange this year. There are four Doctor strange in the trailer and we may see more in the movie, but of course that would be too Strange.
The Strange Dilemma
The Zombie Doctor Strange is not new, but many not know this character since not everyone was interested to watch an animated Marvel series "What If..?". I too thought its just a filler-like series but it has strangely strange connections to Doctor Strange 2.
'What If...?' Is More Real Than Thought
The Sinister Strange and Gargantos are too from the 'What If...?' series along with Captain Carter that we may encounter in the movie, however, Gargantos was defeated and Sinister Strange was redeemed at the end, so it's most likely a misdirect by marvel to hide its main villain while projecting the main Villain as Sinister Strange.
Marvel Is Good At Misdirects
The Zombie Doctor Strange is most likely a misdirect as well, However, it's appearance even in just the trailer is insanely awesome, but the real demon is hiding in the plain sight, this all can't be our Dr. Strange's mistake, there must be someone else pulling the threads.
Zombie Is Awesome But A Misdirect
The Main Villain is hiding in the plain sight and can be caught in the first 10 sec of the trailer, "Every Night, I Dream The Same Dream & Then The Nightmare Begins". Got it Yet? It's "The Nightmare" A demon from the Nightmare/Dream dimension who feeds off fear when you're in your dreams or Nightmare preferably.
Ha! Gotcha... Main Villain!
The movie is directed by Sam Raimi, the same director who's known for horror movies, Doctor Strange 2 is a sequel exploring horrors of multiverse and the beginning of the trailer is indeed like a horror story and a Zombie Sorcerer is there on top of that, and 'The Nightmare' is a exactly the villain who has powers that match the same detail.
The Dots Are Connecting