I was under the impression that there was still a secret faction. Doesn't Rome simply sit in one of the 3 none senate Roman faction slots?
Maybe the secret faction is the one of the famous "occultus" sig or maybe the occultus is nothing but only for fans to make threads about "what-is-the-damn-secret-faction-since-they-said-it isn't-mauryans-nabateans-numidians-bartixians-arvernii......

Only time will reveal what was hidden...