Alexander's Hetaroi
Deity
Of course that wouldn't stop the Arabian player from wanting to convert the foreign cities.Why? Arabia wants to convert foreign cities so that they get more science. Foreign cities want to be converted for the cheap worship building. It works the way it is now. If everyone could build Arabia's worship building without being converted, then the other players would get the bonuses from the building and they wouldn't want Arabia to convert them, denying Arabia the extra science. That doesn't work.
However it would make an interesting dynamic for the other players especially if Arabia got a worship building that people wanted and still wanted to keep their own religion.
Obviously it won't happen but it would be an interesting scenario if that was the case, in my opinion.