Tell him to worry about 2017 first! He'll probably win by for lack of a strong enough opponent, but what achievements could he possibly point to? Guiliani and Bloomberg both had strong records as mayor of a city of 8 million people to run off of. De Blasio does not have anything. The progressives merely tolerate him. The conservatives loathe him.
Given his clear propensity for "progressive" self-marketing at a global level, though, I wouldn't rule it out. But it'd a joke candidacy.
Universal pre-K. A tale of two Americas, just like how it is a tale of two New Yorks.