Q. Is Microsoft Azure Stack the next version of Windows Azure Pack?
A. No. While initially it may seem that Microsoft Azure Stack is Windows Azure Pack v2, they are actually very different, and Microsoft Azure Stack is a new product.
Windows Azure Pack sat on top of various System Center components to provide an Azure-like portal but was built on very different technologies than the public Microsoft Azure service.
Microsoft Azure Stack actually brings the Microsoft Azure technologies to on-premises bringing a true consistency between on-premises and Microsoft Azure and does not use System Center (although System Center will still be used by customers for other management activities). An example is a template that works in Microsoft Azure should also work on-premises using Microsoft Azure Stack.
Note that this does NOT mean there will be feature parity. It will be up to individual teams to decide if certain features are brought over to Microsoft Azure Stack.