This has been suggested before, but I thought I'd gather it into an actual thread.
It seems (to me) like they'd be fairly easy things to add (with the shafts for them-see below-ignored/set aside, for now), even before heating/lighting was implemented, and that they're pretty obvious items to include in a Fortress as "room" items.
A fireplace could be an additional piece of "furniture", that could be added to pretty much any room. Nobles of very high rank might sometimes require one in their bedrooms, dining rooms, and/or offices.
Fireplaces could either be carved out of natural stone, or constructed from blocks (or bricks, whenever they're implemented). Either task could require a mason, and could be further enhanced by the addition of a mantle (which could be constructed from stone or metal, and added as a piece of furniture. The mantle's quality could entirely represent the overall quality of the fireplace, itself, or maybe just add to it.).
If we get shelving (and I hope we do, at some point), a mantle could count as a shelf.
Fireplaces could also require the construction of a shaft leading to the surface, which could be secured by a metal grate. Later on, whenever we get secret doors, these shafts could be disguised. The height of the shaft could factor in to it's quality, since the higher the shaft, the better a fireplace supposedly works.
To use a fireplace could require a log of wood, which dwarfs could burn when they're very cold, getting rid of an unhappy thought. Some dwarfs might also like fireplaces, and burning a log of wood in one might give that dwarf a happy thought.
When we get heating/lighting, a fireplace could ofcourse provide both.
Due to living underground, and their high standards of craftdwarfship, fireplaces could work more efficiently than human-built ones, but an even more efficient method could be to use a stove, instead.
Stoves might have to be built out of metal, or possibly some kind of ceramic. They could provide much more heat, but less (or no) light, compared to a fireplace. In addition to wood, they could also burn charcoal. Kitchens might require atleast one of them, but Nobles might not.
Metal stoves might require a metalcrafter to construct one, and they could be quite valuable. Adding one to a room might enhance it's quality.
Stoves (especially high-quality ones) could be used to heat a much larger area than a fireplace. They could still require shafts, as fireplaces do, and might not give any happy thoughts to individual dwarfs, to balance them out.
Stoves could be separate from ovens. Ovens would be cooking-appliances that could be constructed from brick (whenever we get bricks). The bricks themselves might possibly need to be of a heat-resistant type to work.
Fireplaces and stoves might be used to cook some foods, but others (bread and roasts being major ones) might require an oven.
Fireplace mantles could be decorated (since the inside of a fireplace isn't typically decorated), while stoves could be entirely decorated. Stoves might also be used to heat water, and pipes could possibly be used to channel heated air away from stoves, and maybe also ovens.
Good application of stoves could be used to provide hot running water (not necessarily steam, but better than bathing in an icy mountain stream), warmed floors, saunas, central heating, and maybe even be part of a laundry-service. The heat exhaust could also be used to melt ice for water, in wintertime.