Suggestion: make Clergy a viable option for pagan civilizations.
The easiest way would be to give it a bonus associated with pagan temples. For instance, faster pagan temple construction, or extra production with pagan temples. This bonus should be weaker than the Clergy effect associated with state religions, to encourage adopting a religion, but it should make Clergy competitive with Deification for pagan civs.
My rationale is that as long as you're pagan, there is zero strategic choice for religious civics, which is not interesting. Although Deification—i.e. where the king is a godlike authority over the local religion—is probably sufficient to represent most historical civilizations with an elaborate pagan religion, there are cases of pagan civs where the priestly class seems to have been somewhat separate from central political power. The Maya are an example, and arguably the Aztecs (the Aztec emperor was a priest, but as far as I understand he was only a priest for one of the gods, and wasn't deified). So players would be able to choose whether to emphasize central religious power or a the existence of a priestly class.
Clergy could also be used to ease the transition from paganism to a major religion once one has spread. Instead of converting to the religion and then have to switch out of Deification, or switching to a useless civic first and then adopt a religion, you could switch to Clergy, enjoy its (weak) benefit for your paganism, and then adopt a religion.
The easiest way would be to give it a bonus associated with pagan temples. For instance, faster pagan temple construction, or extra production with pagan temples. This bonus should be weaker than the Clergy effect associated with state religions, to encourage adopting a religion, but it should make Clergy competitive with Deification for pagan civs.
My rationale is that as long as you're pagan, there is zero strategic choice for religious civics, which is not interesting. Although Deification—i.e. where the king is a godlike authority over the local religion—is probably sufficient to represent most historical civilizations with an elaborate pagan religion, there are cases of pagan civs where the priestly class seems to have been somewhat separate from central political power. The Maya are an example, and arguably the Aztecs (the Aztec emperor was a priest, but as far as I understand he was only a priest for one of the gods, and wasn't deified). So players would be able to choose whether to emphasize central religious power or a the existence of a priestly class.
Clergy could also be used to ease the transition from paganism to a major religion once one has spread. Instead of converting to the religion and then have to switch out of Deification, or switching to a useless civic first and then adopt a religion, you could switch to Clergy, enjoy its (weak) benefit for your paganism, and then adopt a religion.