According to an anonymous reader on Destructoid, an online survey asking readers to choose their favorite box art image for Epic Mickey 2 strongly suggest the existence of the game, though it has to be officially announced.
The survey also asks which title out of four would be best: Epic Mickey 2: Return of the Mad Doctor, Epic Mickey 2: The Power of Two, Epic Mickey 2: Mystery on Mean Street, and just Epic Mickey 2. So that suggests that there is Mad Doctor, a second character (likely Oswald The Lucky Rabbit from the box art), and a street that isn't nice…
If this is true, let's hope the game actually lives up to all the hype this time.
[Source]