I have a sugestion regarding multitenant Azure Stack
Will switching tenant directory in the tenant portal be available soon or at all?
The directory filter button in the top right corner of the tenant portal would suggest that one would be able to switch tenant directory and subscriptions

Scenario adam@contso.onmicrosoft.com has a user subscription on Azure Stack and can login to the tenant portal and work with Azure Stack.
adam@contso.onmicrosoft.com user is also invited as a guest user in fabrikam.onmicrosoft.com AAD and has been assigned a subscription via that admin portal to with tenant directory fabrikam.onmicrosoft.com specified,
or has been added via another fabrikam.onmicrosoft.com native users to their subscription via AIM.