Let's face it, when it comes to villains, Dr. Strange has a very limited pool of enemies so it can be kinda easy to figure out who he will be going up against in new film. Dormammu and Baron Mordo are his top 2 villains and without a doubt Mordo and Dormammu will be the main villains. It just depends if the studio is planning on building up to Dormammu in a series of filme or have him be right there from the start.
Personally, I would love to see Dr. Strange go up against the Demon Lord Nightmare. They could do all sort of neat stuff with a dream world, nightmare illusions, his demonic black horned horse Dreamstalker and they can get a really creepy actor to play him like Willem Dafoe , Cillian Murphy or Ralph Fiennes to play him.
Personally, I would love to see Tim Curry or Sir Christopher Lee in the role but sadly Sir Christopher Lee would be considered to old (unless he did the voice and they used computer animation to capture his facial expression like how they're doing Ultron) and Tim Curry would be considered too...let's say rounded for the role.
Of course the studio could do the opposite and create a new villain all together for the film.
I just hope they do everything they can to make the special effects look as much as possible like Steve Ditko's art work for the comics.