It needn’t be that way though. Not only are there societies that could be added, but other things can be done to add to the game early on that allows moving anachronistic entities back to their proper time periods. For example, the way that the reformations of the pagan religions are being treated in Holy Furry

D) can be done for religions like Christianity and Islam, as well as others. Islam was still very new and giving ‘decision avenues’ that would mold the religion going forward and maybe even creating your own heresies would be not only interesting but a practical reality in simulating this time frame. As it is the game treats Islam as an immutable finished product with a few clear heresies that come down the pike. In a game that changes history it needn’t be only the pagans that might have changed.
Also, once you give proper historical time entries to the religious movements like the Druze, you can also add a start option to randomize the established heresies. This way people who want accuracy can have it but those who want the flavor of lots of different religions, even if anachronistic, can have that instead.
Then there are things like the Arians. The last Arian King’s we’re gone in the 7th century, but the concept of Arianism was probably still in existence when the game began. Consequently, they could be added as a fading group for the 769 start and more can be found I am sure..... Hmmm, I think I need to contact some old friends who are medievalists.