The exact release date of the developer preview will depend on a number of things. First and foremost it will depend on the feedback we receive from the upcoming demo installation we'll be setting up this year. There's also one or two things we'll need to finish off behind the scenes (mostly Admin CP stuff which you won't see during the demo installation).
If all goes well, it could be very early in 2017. Depending on how that goes, there will likely be other preview releases, such as Beta and RC releases all the way up until the final release. Again, the arrival of each stage of the process will depend on how long the previous stage of the process took.
I feel now is probably a good time to re-iterate something about the developer preview. You will not be able to upgrade to it from an existing XF 1.x install. You will not be able to upgrade it to the final 2.0 version. You will not be able to import from/to it. It will strictly be for development and testing purposes only. All I'm saying is, don't start trying to upgrade your live XF 1.x installs to it, or start trying to launch a new 2.0 forum on it