If you don't like the idea of flame cannons, how about just adding oil and lamps?
It is easily explained in terms of the mythos, as oil could either be mineral (in which case it could be presented gameplay wise as pockets of fluid found in rock; "xdwarf cancels dig, found oily rock" anyone?), or animal/vegetable (as an booze-like substance made by rendering down fat possibly?). This would then allow you to either a)flood an area with oil as you would water, which instead of depositing mud, would deposit !oily slick!, or b)you can 'build' an oil slick, maybe using one barrel of oil for a 3x3 area for example.
Once this slick is deployed however, you can tell the dwarves to ignite it, whereupon the nearest dwarf will grab a lamp or torch and hurl it onto the slick => BAM instant goblin flambe.
Further, when the torches/lamps aren't toasting little green men, they could be installed as light sources in the wall of your fortress, which would tie in nicely with Toady's goal of the whole darkness/lighting upgrades.
Also, we need blackpowder. Not in guns or cannons, but as blasting charges (say, removes a star shaped chunk of material in x, y and z directions), which might then finally make the alchemist's labratory a worthwhile building.
Oh, btw, I'm a new poster, but I've been playing & lurking for a while. Hello everyone.
[ February 28, 2008: Message edited by: Osmosis Jones ]