With Threshold, my sources say, there could be three primary SKUs: A "modern" consumer SKU; a traditional/PC SKU; and a traditional enterprise SKU.
The modern (i.e., Metro-Style/Windows Store) consumer SKU
would be focused on WinRT apps. (WinRT, in this case, refers to the API set at the heart of Windows, not the current Windows RT operating system that runs on ARM.) It may end up targeting ARM- and Intel-based devices both. It would be updated frequently by Microsoft through the Windows Store.
This modern SKU would be the SKU for Windows Phones, ARM-based Windows tablets/PCs, phablets and other kinds of tablets. Some PCs also may run this SKU, providing Microsoft with a more head-to-head competitor to Chromebooks, as these machines would be more secure and locked down.
A more traditional consumer SKU
would be aimed at the current PC market. This SKU would include a desktop and be customized so that mouse/keyboard users will be able to continue to have some semblance of productivity and familiarity with Windows. This SKU also would be updated regularly and often through the Windows Store.
There also will likely be some kind of traditional Enterprise SKU,
according to my contacts, that would include all the usual business bells and whistles, like support for Win32 apps via a Desktop environment, support for group policy, device management and more. This SKU would be aimed primarily at traditional PCs, tablets and other devices and also allow users to run "Modern"/Windows Store apps.