We've seen hints of that in the works for the last few years...
Continuity/handoff is the final software piece of the puzzle.
- Add iOS-like features to OSX (notification center, launchpad, etc.)
- Get OSX and iOS to look visually similar
- Dumb-down OSX apps to attain feature parity with iOS versions (iWork in particular)
- Swift programming language
All of the software pieces are in place to create a true hybrid device that operates as a notebook when attached to a real keyboard/trackpad and continues on in tablet mode when detached. Not by mashing elements of a desktop and mobile OS together, but a synergy between 2 OSes that are specialized for their respective tasks.
I've always believed that Microsoft saw this coming and rushed the Surface out to market in an attempt to establish a foothold before Apple unleashed this beast of a device.
To a lesser degree, the very minor (IMO) update of the iPad Air and iPad mini might be the calm before the storm as Apple focuses resources on this hypothetical iPad Pro.
I love my 11" MBA, and iPad 4 (though much less after upgrading to iOS 8). If Apple were to produce a device that combined those two into a seamless experience, I'd be all over it. Though I might be in a very small minority.
You might be in a very small minority at first because frankly people are afraid of a major change like this, but once people realize how much better of an experience this is, that will change. Only the diehards who are somewhat stuck in the last century or at the very least the last decade will continue to moan for a while. Even they'll get it eventually.