I'd like to see a major D&D campaign setting featuring just one Church, with one God. Druids would be leftovers from an older time, and might even be persecuted. Inquisitors and witch burnings would be commonplace. Penitents and Faithful Knights would also be seen fairly regularly. All knowledge would be reviewed by the Church, who would be the resident teachers throughout the land. Mages would still exist, but either work for the Church or be independent scientific types.
Saints would exist, and might even receive worship as part of the services to the God. There might even be a living Saint somewhere in the world, who would act as a beacon to the faithful. Miles-long pilgrim lines would form at the living Saint's church, with people being blessed just by his presence. Priests would have a saint's finger bone as a holy symbol, and every church would have the bones of some saint interred inside the altar.
Also, there would be no racial gods. Evil races such as orcs might worship demons, and have demon-summoners among their ranks. The demons would also be able to grant spells to their followers. Demon-worshiping humans would be hunted, but very definitely present. Succubi could hide themselves in a King's Court with none the wiser. There might even be a very powerful mage with demon servants, that even the Church cannot fight, as he has his tower in a major city, and trying to kill him would likely wipe the city out.
I know WFRP has some of this vibe, but I'd like to make it even more like the Dark Ages of Europe, without just being a historical fantasy game.