Android "M" already talked about by developers59
The discussions that mentioned Android "M" aren't anything that Android users would find interesting, although developers might. Based on the time between the introduction of KitKat and the expected release of Android "L", we could see Android "M" get pushed out sometime at the end of next year or early in 2016.
You can check out early discussions about Google "M" by clicking on the slideshow below. In keeping with Google's naming policy for different builds of its Android OS, what do you think the "M" will stand for? Marshmallows is one guess.
source: AOSP via MYCE