I think it would be an intriguing addition to the game to make it possible for someone to found the secret society equivalent of a religion. Basically, think secretive cult. I think a few small changes could make this possible while utilizing the existing system.
1. Allow smaller temples and shrines to be hidden from view.
Basically, someone building a new shrine or temple could spend extra gold to make it "hidden". This would prevent anyone except for followers of the religion from seeing it. If the temple were expanded beyond a certain relatively small size (say a primitive temple, or possibly smaller), it would automatically lose its secret status. Or, perhaps it has to be torn down and an official temple has to be built from scratch to make it public. As a trade off, they cost more and would have a reduced effect on conversion rates. Someone in the region who suspected the presence of such a temple could attempt to find it with a unit of men. If successful, it could be sacked.
2. Allow secret practitioners.
One thing that bothers me about the current system is how the game seems to assume that religious affiliation is public knowledge in one specific case. If you are the lord of a region with peasants who belong a majority religion that has decreed your faith to be evil, the game operates as if the peasants know of your affiliation. Every now and then in the Daily Region Report, you get a line that says "The peasants are unhappy that their lord belongs to an evil faith" (or something like that). This can have the effect of outing you to your entire realm. If you have joined an enemy realm's faith, this can create problems for you. If I'm a member of a faith that I know full well is considered evil by my countrymen, why on earth would I make it public knowledge? I'd practice in secret, of course. I'd suggest adding an option in the politics or command section allowing you to hide your religious affiliation. The religion doesn't get the small bonus to conversion that accrues from having you as lord of a region, but you don't get publicly outed.
3. Allow secret sermons.
A priest would be able to preach secretly. No announcement would be made to nobles nearby, but at the cost of a significantly reduced effect on conversion.
The net effect of this would be to allow unpopular religions to worm their insidious way into unsuspecting realms more easily. This can range from efforts by an established and very public religion to infiltrate a hostile realm to the creation of a religion that is designed to be more cult-like and sinister with entirely hidden temples and a hidden agenda. Naturally, if all your temples are secret they will also be small and you won't get converts easily (private sermon only), but that's how something like this *should* work. I also think the potential atmosphere of paranoia and secrecy could be very rewarding. You'll see that a heretical religion is gaining followers in some of your regions, but you can't prove anything... Yet. Do you send troops into the regions to search for secret meeting places of banned religions at the risk of alienating your lords and being embarrassed if nothing is turned up? How do you know which lords have been turned?
Thoughts?
