Lets hope W10 will be similar.
I'd wager similar to the OS X scheme - 10 will stay the major version number, and instead increments on minor and build numbers going forward as a possibility.
You know there is a way they can make money this way. You get the base windows 10 for free, but if you want some extra features you pay extra.
I was thinking more along the lines of Xbox connectivity, holographic capabilities, accessing company domains, data encryption.
That sort of thing.