I recently read the Hall of Legends archive of Nist Akath, and that, along with the inclusion of vampires, necromancers, ghosts, zombies and skeletons, turned my thoughts toward the traditional fantasy counter - gods, and those who worship them. And so, I spent some time contemplating the issue, and came up with this. (Note: My ideas are mainly aimed at Fortress mode - I really haven't done much with Adventure mode.)
While I have no idea how difficult it would be to add a dwarven desire for worship, it should, in my opinion, be a "want" rather than a "need". It should be the kind of thing that dwarves will make use of if it's available, like a statue garden or zoo, instead of a hard requirement, like food & drink. A lack of religion should not generate bad thoughts, as dwarves can always toss off a quick prayer to Armok as they labor. At the most basic, religion would provide a small amount of happy thoughts. With much effort, it could also provide players a way to help combat evil.
Also, I'm not sure if every dwarven diety should be available to every settlement, or if there should be regional dieties based on world generation. I don't even know if they're set for every world, or randomly generated - I've honestly never paid attention to them, other than noticing the occasional diety on a dwarf's relationship page.
On to the details:
The Altar
An altar is an item of furniture, and follows all of the traditional rules of stockpile storage and placement. Creating an altar is a two part process - a rock table is crafted in a Mason's Workshop, and is then engraved in a Craftsdwarf Workship. When the "Engrave Altar" job is selected, a menu of all available deities will be shown, along with the number of altars already available for each deity. The quality of the altar will be based on the quality of the table used, the engraver's skill, and may be further enhanced by being encrusted with gems. Placing an altar is as simple as placing a table - select Altar, the location, expand the menu to choose a specific altar if desired, and place it.
Dwarves will pray at an altar as they desire, which will generate a +1 happy thought (on the scale of 1-10). If they actually worship the deity the alter was engraved to, they will get a bonus +1 happy thought. Happy thoughts will be modified by the quality of the altar.
A simple altar will only be usable by a small number of dwarves per (week? month?) limited time period. Perhaps it would only be usable by one dwarf at a time, like a 1-tile pond.
The Temple
A temple is a room designation, that is defined from a placed altar, in the same way a dining room or bedroom is designated from a table or bed. The advantages of designating a temple are three-fold. First, the quality can be enhanced by placing other pieces of high-quality furniture in the room or by engraving the stonework, which boosts the happiness gained. Second, more dwarves will be able to pray at the same time. Third, and probably more important, is that the room may be assigned to a dwarf, who will then become a Priest (or Priestess).
Priests & Priestesses
A priest is created by assigning a dwarf to an altar, after a temple has been designated from that altar. Priests will show up on the noble screen, and would be on about the same social level as military captains. Requirements would be simple, and they would generate no demands. Instead of creating another job (such as Mason), they would simply use some social skills, like the Mayor, Broker & Bookkeeper do. Possible skills used might be Organizer, Consoler, Conversationalist, and Judge of Intent. Possibly also Pacifier, Intimidator, or Comedian, based on their deity.
A priest will tend their temple, organizing worship services. This will result in more dwarves being able to satisfy their religion want than an untended temple would, and also give a boost to the happy thoughts generated. Another important task would be to hold a Funeral or Requiem service. Funerals would be held after a dwarf's body has been interred, and would help ease the suffering (bad thoughts) of dwarves close to the deceased. A Requiem service would perform a similar function after a Memorial Slab has been created & placed, when a corpse is lost or not able to be recovered, but would not be as effective. The final task of an ordinary priest would be to hold a special Requiem in case of a haunting. This would temporarily lay a ghost to rest, perhaps only for six months.
I would imagine all of these to be automatic, although maybe the player should be able to order funerals & requiems, in case they have a lot of deaths. Multiple requiems for the same deceased dwarf would have rapidly diminishing returns, although they would still count as worship services. Multiple priests could be assigned to the same altar, although only one service could be going on at once. There might be other types of activities available, such as simply talking to fellow worshipers, or actively converting dwarves without a deity.
The High Priest/Priestess
Once a settlement is large enough (qualifies for Barony?), this new noble position would be available. A High Priest has bigger requirements, requiring a grand temple to their deity, as well as grand rooms. (About on the level of a Baron's requirements.) They might generate demands, although I haven't thought much about what they might be. This noble would also be very useful, unlike some others.
A High Priest will occasionally organize a worship service, which will give a larger boost to happiness, and having one appointed will also generate a trickle of happy thoughts from the nobles, as they don't have to attend worship with the common rabble. Most of the time, they will wander around, like a stuck-up noble. But the main advantage of having a High Priest lies in their other (player ordered?) jobs...
Exorcism - engage in spiritual combat with a ghost, permanently "killing" it. This would generate some unhappy thoughts from friends & relatives of the deceased, as the soul was destroyed, not laid to rest. (But it'd be great for those pesky caravan guards!)
Sanctify - hallowed ground would prevent, or slow, the creation of undead on it. It might also cause damage to undead, or simply act like a restricted/low usage travel zone. I thought about making this "cost" amulets or figurines to create - say, one amulet per 3x3 area. It would also be a temporary blessing, fading after 6 months or a year. This would prevent the player from sanctifying the entire map, and give a reason to keep making crafts. I imagine the process as more like building a farm plot, instead of placing a zone - once it's set by the player, the High Priest has to go "work" at it to actually create it.
Vampire Hunter D(warf) - suspicious activity might set a High Priest on the trail of a vampire. Perhaps they could identifiy the creature, or allow it to be punished/destroyed. I have no experience with vampires yet, although I just had a missing dwarf found "drained of blood" in my current fort, so I have no idea how they are revealed or prosecuted.