I know they had something similar to this in Knights of the Nine, but I think they should implement a Cleric type of guild as a main faction and go alot deeper into the concept.
(EDIT: Something that should be noted is I have not played Morrowind, only Oblivion and Daggerfall. I was not aware that religious themed guilds had been implemented. I'm not only purposing religious themed guilds but also the possibility of an entirely different style of play.)
What I have in mind would be essentially the opposite of the Dark Brotherhood. Rather than getting tasks to kill nobles and innocent people, you would get tasks to heal and protect townsfolk.
Now I know alot of the warriors and mages guild quests are geared towards good deeds rather than bad, but I see the Clerics guild as focused more on healing and, for instance, purging a town of a deadly plague by curing everyone of the disease without catching it and dying yourself. This way there would be a way to be a master in the craft of restoration magic, making it much more versatile and in depth than just using heal spells as a utility for yourself when you are low on life.
The Clerics guild wouldn't be devoid of combat either, it would just be geared towards killing/protecting others against Daedra, Undead and Vampires.
(EDIT: The idea of a religious fanatic guild is also included in the poll, but not limited to it. The example I'm giving is the opposite of an evil guild, but there could also be Church guilds doing evil in the name of good, or their god.)
So what would the Guild offer that the other guilds don't? Well, it would be very similar to a mages guild for the most part, but focused more on restoration and alteration, so you could buy and make more powerful healing and shielding spells for better prices than the mages guild. There could be unique spells that deal extra damage to evil creatures like Daedra/Undead/Vampires etc. And maybe the clerics guild could be the only place to buy or create an AoE heal and enchant weapons with extra damage vs. evil creatures.
As you advanced in rank you could get robes and other items that make you immune to poison and diseases, eventually being totally immune to the disease that turns you into a vampire and the such.
I know some people aren't very intruiged by the idea of being primarily a healer, but trust me, I know it could be implemented in a way for it to be fun and still fast paced action. It would just be a different aspect of play. I'm not trying to suggest some sort of MMOish feel, I know alot of people aren't big on staring at a bunch of bars but this would still play the same as any other Elder Scrolls game, just from the point of the pious one.
What do you think?