There's actually a road map hidden in plain sight, included with a recent optional update for Windows 7 and Windows 8.1. An XML file installed with that update contains important clues about a program called GWX: Get Windows 10.
KB3035583 describes itself innocuously enough: "This update enables additional capabilities for Windows Update notifications when new updates are available to the user. It applies to a computer that is running Windows 8.1 or Windows 7 Service Pack 1 (SP1)."
Get Windows 10: Microsoft's hidden roadmap for the biggest software upgrade in history | ZDNet
The above is a few paragraphs down into Ed Bott's article, but for me is the real purpose of letting us know what's next.