Well... Let's look at what we know.
• WoW: Classic was likely announced way too early. This has been supported by what they've said in interviews.
• They have a LOT of work ahead of them. Most of this is because they have to build a lot of the game from the ground up to work with the new architecture.
• Blizzard is notoriously slow at some of these projects. Diablo III, for example, was supposedly in the works from 2001 onwards, but didn't receive any official announcement until 2008. Even from that point forward, it wasn't released until 2012.
Idk, this one seems to me like it will take a while. I have no doubt in my mind we won't see WoW: Classic this year at all. My bet is late 2018 at best.