Dracosolon
Prince
What if the "one less Temple required per Cathedral" effect is given to the Clergy civic? I think it could be synergetic with a lot of medieval and early modern states that built a lot of religious architecture. Can be given to both Clergy and Theocracy, if also possible.
China: 2 Confucianism, 2 Taoism
Ethiopia: 1 Orthodoxy
Korea: 1 Buddhism, 1 Confucianism
Poland: 3 out of the three Christian religions
Mughals: 3 Islam
Mexico: 3 of your state religion
One possibility would be to restrict it to Catholic civs by having it require an Apostolic Palace resolution. That way the change is restricted to Poland (1 less temple, still has to possess 4 cities) and Mexico (the UHV could be amended to 4 Catholic Cathedrals specifically, though that would require Mexico to always start Catholic, and also requires 1 more Cathedral in a limited timeframe. Or the Apostolic Palace effect could obsolete/not apply to the New World).
Thinking back to this discussion, I've tought of an alternative: Why not allow state religion cathedrals with one less temple if you're on the same continent/subcontinent as the shrine? By this I mean the categories below, excluding Africa, Asia and America:
lEurope = [rBritain, rIberia, rItaly, rBalkans, rEurope, rScandinavia, rRussia]
lMiddleEast = [rAnatolia, rMesopotamia, rArabia, rPersia, rCentralAsia]
lIndia = [rIndia, rDeccan]
lEastAsia = [rIndochina, rIndonesia, rChina, rKorea, rJapan, rManchuria, rTibet]
lNorthAfrica = [rEgypt, rMaghreb]
lSubSaharanAfrica = [rEthiopia, rSouthAfrica, rWestAfrica]
lSouthAmerica = [rBrazil, rArgentina, rPeru, rColombia]
lCentralAmerica = [rCaribbean, rMesoamerica]
lNorthAmerica = [rCanada, rAlaska, rUnitedStates]
lOceania = [rAustralia, rOceania]
lAfrica = lNorthAfrica + lSubSaharanAfrica
lAsia = lMiddleEast + lIndia + lEastAsia
lAmerica = lSouthAmerica + lCentralAmerica + lNorthAmerica
This would effectively extend it to other religions than just Catholicism and be a bit fairer. Some UHVs would be affected though:
Poland: one less Catholic or Protestant church depending on state religion (I'm assuming that the Cathedral is destroyed if the requirements are no longer met so you can't just switch state religion - alternatively, that's one less Catholic and one less Protestant church), still needs four Orthodox ones.
Mexico: unaffected.
Ethiopia: unaffected unless a city in Africa is chosen as Holy City (which could be prevented if the Ethiopia player is human I guess?).
Mughals: affected if the requirement is simply continent instead of subcontinent.
Korea: affected for Confucianism, same deal as Mughals for Buddhism. If the "Cathedral is destroyed if its conditions are no longer met" rule applies, then that's just one less temple.
China: affected, but that means the Chinese player has to spend two

Note that some of these require an AI to do you the courtesy of building the shrine for you, which means you might still want to plan for one more temple rather than rely on luck.