I'm just going to brain storm here a bit and just type whatever comes to mind which seems like a good idea....
I think natural gas would be a awesome feature for the game.
Few of the things you can do with natural gas...
#1 (When lighting is implemented) All the torches in your fortress can be linked to a natural gas line. (Otherwise all the torches would have to be refilled all the time, if thats how its planned to be)
#2 (Do dwarfs get cold?) if they do, or if they will at some point in the future, Natural gas would be a great way to heat up rooms.
#3 Flooding your entrance room with natural gas would/could knock out any creatures in that room. Rather then flood the room out and killing all the invading enemy's in there,(also i would think its possible that one of your own dwarfs could get stuck in there when you hit the lever for the flood gate), well then also your dwarf would get knocked out as well, sparing his life.
In fact natural gas could be a fail safe in a way, in case one of your dwarfs get caught in there by accident. You can just gas the room, but also leave in the options for flooding and magma
Ventilation.
Ventilation shafts would be used to carry natural gas to different areas.
Ventilation shafts should be build able only in walls. The walls don't need to be removed to have a shaft in them.
Valves and other such things can be made which hang off the walls or connect with a wall with a ventilation shaft in it, to control flow.
Ventilation shafts could also be used to carry heat from a furnace or a magma chamber, how ever some sort of "Bellows" which can be hooked up to a powder source would be needed to blow the hot air through the ventilation shafts.
Ventilation shafts should only be build able in a wall next to a dwarf, IE it cant be built 2 tiles away from where the dwarf can reach.
A Ventilation shaft could as well be built from your Refuse room to the outside to air out any miasma that comes about, or the butcher shop as well, which seems to get it once in a while.
I don't know how possible it is that either of these ideas will ever come about in the game, but i think they would be awesome features if they can be added in.
I'm pretty sure from reading AD&D fantasy stuff that dwarfs used natural gas?
Also i'm sure theres plenty of other things which Natural gas and ventilation shafts could be used for, i only started with ideas. I could go on for hours... But i'm not