There are some reasons to believe that Apple will be abandoning Open Directory on their OS in future versions. Right now you can manage it through the Server.app on Mac OS X. And while the grandaddy and business favorite of external authentication is Microsoft's Active Directory, I have had a number of clients wanting to go all Mac and did their directory services on a Mac server.
First of all, there is no public information or announcement from Apple saying there are going to do this. But if they do, it may impact a number of FileMaker solutions that tried to do everything on Macs. The obvious solution is to get a Windows Server and start creating groups and managing them on a Windows Server.
Then again, maybe the solution is more moving towards OAuth. FileMaker has made its first foray into OAuth with FileMaker 16 (Amazon, Google and Microsoft Azure). Granted we still have a lot of limitations such as only these 3 OAuth services work and you can't even script creating an OAuth account in FileMaker and it has to be done manually. Then again, this may make Microsoft Azure even more attractive since it integrates Active Directory into OAuth making the network groups available through OAuth authentication.
What would be other impacts on FileMaker Server if Open Directory external authentication went away on the Mac and what different or better solutions are out there?