I would suggest that you only can create an empire when there is no emperor from your religion/heresy.
There is some historical ground for that, the Orthodox Russians claimed their imperial title only after the fall of Constantinople. Having only a Catholic Holy Roman Emperor, a Orthodox Roman Empire, a Shia Persia/Iran and the Sunni Caliph, and of course the Tengri Genghis Khan for most of the Time period.
With the exception of some Spanish king trying to claim to be emperor, but that wasn’t recognised outside his realm.
And the Latin Empire was more a usurpation of the Roman Empire
There is some historical ground for that, the Orthodox Russians claimed their imperial title only after the fall of Constantinople. Having only a Catholic Holy Roman Emperor, a Orthodox Roman Empire, a Shia Persia/Iran and the Sunni Caliph, and of course the Tengri Genghis Khan for most of the Time period.
With the exception of some Spanish king trying to claim to be emperor, but that wasn’t recognised outside his realm.
And the Latin Empire was more a usurpation of the Roman Empire