I'd say little bit like in Civ IV.
Every single religion is equal, but they are real historic ones, choice is for the player to make. Buffs to happiness and culture. Extra bonuses for cities following state religion. Interactions with buidings, wonders and civics like usual.
And if you dont care about religion at all you can go for free religion from paganism.
I did enjoy Secret Societies quite a lot actually. It was pretty much always on for me in Civ VI. Just gave that little gentle prod in some direction when it was hard to choose what to go for. Sometimes it didnt do too much. I think a vampire now and then helped AI to defend a little bit better from my invasion too that made things more interesting. I would not mind something like that either.
Balance is what I am worried about, Firaxis has to nail it better I think if its so. Also I always like it be more realistic and historic. Secret Societies is actually odd ball in my taste I guess.