Migrating On Prem AD to Azure AD and doing away completely with On Prem AD

If you do not have lot of group policy dependecy then upgrade to AD premium p2 for joining device to AAD or ntune management.

If you would like to use AADS as managed domain controller then you may have to build another registered domain like .org or .net built trust with .local domain and add domain suffix for azure AD domain the syncs to cloud identity. In other words you are still managing two namespaces until you completely move identity to cloud with intune instead of GPO.

AADS does not support schema extension or sync, it will be created with two OUs initially and you will have to built rest manually and apply policy probably from CSV import or xml import/export or add manually and install azure ad connect server on cloud to sync.

I am planning same considering I do not have lot of group policy for stand alone mac users. I only have to figure out joining VMs directly to Azure AD and not go through building cross forest trust just to survive on old GPO. 

First I am looking to convert all server VMs to as much as PAAS solution and work on identity management to AAD after I build complete cloud presence that way I am not reliant on prem hardware when switches of firewall goes down. user can still connect with wifi