The two big questions I have about using Azure AD Application Proxy is as follows:
Can the Active Directory in Azure be used for outside emails aka (@gmail, @yahoo, ect. ect.)
Will the people picker be populated with using Azure AD Application Proxy? This is a show stopper.
Stay tuned because if this is not answered we are having a POC to try to accomplish this. Yes we know we will have to pay for the per user with Azure AD Premium.