According to the rumor mill, the Start Menu will be the entire Start experience in the desktop and laptop oriented version of Threshold, there will not be a Start Screen but you will be able to maximize the Start Menu so that it takes up the whole screen and works in a similar fashion like the Start Screen does today in Windows 8.x.
The new Threshold Start menu isn't based on the old Start menu. It is instead a new Start experience based on the Windows 8.x Start screen. It will likely auto-detect your PC type on first launch, and if the system boots to the desktop—as it does on traditional PCs in Windows 8.1 today, it will provide a menu that is roughly analogous, size-wise to the Windows 7 Start menu. But if you are using a tablet or other touch-based system with no keyboard, you will boot to a full-screen Start menu that looks and works a lot like the Start screen does now. Users will be able to toggle between the two displays, I'm told, and configure it to work as they prefer.
Source: SuperSite for Windows