Marvel has been working on a single movie for Doctor Strange for the past couple of years and Collider suggests that he could appear in Phase 2 and have his own movie for Phase 3, a phase which they feel is inevitable.
If you know nothing about Doctor Strange here is a basic biography for him:
"Born Dr. Stephen Strange, a world-renowned but selfish neurosurgeon who only cared about wealth from his career. A car accident damaged his hands, shattering the bones. The damage and subsequent healing however, effectively ended his ability to conduct the surgery as he had before since his hands now tremble uncontrollably. Too proud to take on a teaching job, Strange desperately began the search for a cure to restore his hands, consulting various doctors, homeopathic treatments, and traveling around the world to remote regions for exotic cures, all to no avail.
Having completely exhausted all funds and reduced to homelessness, Strange is left to performing back alley medical procedures for cash. Depressed and still searching for a cure, Strange locates a hermit called the Ancient One who refuses to help because of Strange's selfishness, but senses his good side which he attempts to bring to the surface. He fails but Strange's goodness does show itself when he discovers the Ancient One's disciple, Baron Mordo, attempting to kill the old man. After Strange selflessly thwarts Mordo (who becomes Strange's most enduring antagonist) he is taught the mystic arts.
As the Ancient One's new disciple, Strange encounters the entity, Nightmare, and a number of odd mystical foes before battling his eventual arch-foe: Dormammu, a warlord from an alternate dimension called the Dark Dimension."
Given the information we have on Doctor Strange I wouldn't be surprised to see him surface at some point in Phase 2. However, I think shoving him into Thor: The Dark World would be overkill. The film already has two villains (3 if you count Loki) and I think that would be a little bit too much. If they are going to throw Doctor Strange into Phase 2 I would prefer him in Guardians of the Galaxy.
Also bear in mind that this is just a rumor. There will be and has been no shortage of Marvel casting rumors so don't take anything too seriously. The only reason this has broken out of its shell is because the Twitter account that hinted the news was right about Falcon being added to Captain America: The Winter Soldier and that Sebastian Stan will return as Bucky/The Winter Soldier.