As we are approaching the 2.0 release, we need to start thinking about the 2.1. Previously, I have advocated for holding 2.1 for traitlets and the GUI refactor, however given the time delays, this needs to be re-addressed. There is already a tremendous amount of work sitting on master that we need to get out to the world (for example story645's work on plotting against string-categoricals).
To start organizing for this I have created a 2.2 milestone, please move any issues that you do not think should block 2.1 to 2.2. Things that you can not be done with out a major API break, please move to the 3.0 milestone.
I will commit to reviewing the GUI overhaul (once 2.0 final is out) so that should stay in 2.1 (sorry oceanwolf and fariza!).
There should probably be a wider discussion / rethink of how we use milestones/projects, but I think we should defer that until github's current rash of UI / PM changes settles down.