Q. Is there a way to federate Azure AD to on-premises without using ADFS on-premises?
A. Microsoft extensively tests and supports federation to Azure AD to on-premises Active Directory using ADFS and where possible utilizes this approach. However, it is also possible to use other federation solutions that support WS-* based identity providers such as WS-Trust and WS-Federation. Microsoft has a number of pages giving detail on this.
- Office 365-Identity program - http://blogs.office.com/2013/09/03/works-with-office-365-identity-program/
- Third-party identity providers - https://msdn.microsoft.com/en-us/library/azure/jj679342.aspx
Note that using these solutions would require extensive testing to ensure the functionality meets requirements across the many different ways Azure AD can be utilized.