Hybrid Handoff
Will Apple ever make a hybrid tablet / desktop computer?
Traditional mouse and keyboard user interfaces don't work well on touchscreen devices, and apps designed for touch don't work well with a traditional user interface. They are fundamentally different, so much so that Apple decided, back when first designing the iPhone, to build a completely new UI framework and paradigm for touch-based apps.
A lot of the "can you do real work on the iPad" debate boils down to input. Touch is better for some things, but for anything that involves creating or manipulating a lot of text, a physical keyboard and mouse is better. There are other issues, like screen real estate and the extra widget density afforded by not having to have such large targets for each button, but in my opinion, mouse and keyboard vs a touchscreen is the main differentiator.
It's not as simple as using a keyboard to type text into an iPad app, because text editing involves a lot more than just typing. Selection, cursor movement, document navigation, these are all things that you expect to be able to do with the keyboard. Many users learn keyboard shortcuts for common operations, and can work almost entirely using the keyboard.
Apple has both traditional and touch-oriented versions of all their major applications. They also have a way for these applications to seamlessly share documents, through iCloud.
And with Handoff, in Yosemite and iOS 8, they now have a way for a user to switch between a touch-based app and a traditional app without losing their place.
The iOS simulator is a complete execution environment for iOS apps, that works on top of Mac OS X. Developers use it to develop their apps, so every iOS app can run on a Mac, in the iOS simulator environment. Convert this simulator into a real runtime for iOS apps on the Mac, and you've got the start of a workable hybrid.
Imagine a Mac that had two displays, with one of them being a touchscreen. Run the iOS environment on the touchscreen, and run the traditional Mac OS interface on the other screen. Put one of these touchscreens as the top lid on a MacBook Air style device, and the other as the inside display.
You could use iOS on one side, and Mac OS on the other side. But the really interesting thing is how you could switch between these two environments just by opening or closing the lid of the computer. Start working on a document in Pages for iOS, open the lid, and the document appears in the desktop version of Pages.
One of the missing pieces is security. iOS devices are locked down, to an extent that they could never lock down a Mac. But what if...
There have been rumours of an A7 or A8 based Mac. So let's say they build that, but for this new Mac, only sandboxed applications are allowed. No user-installable device drivers, no acquiring apps outside the app store. It works just like iOS. Now you've got a Mac that's as locked-down as iOS. It doesn’t have to replace the Mac line; it would be a new category of device.
I'm not sure Apple would ever go there, but I find it interesting how close they are.