Well, we've already got tech levels in the game in terms of what smelting you can use. I think once the physics model is down, it's just a matter of enabling or disabling the constructions that allow you to take advantage of them (this for electricity and steam power). I think people are less concerned with making steam pressure build than they are a turbine device to harness it.
Gunpowder is already possible, if you count a smelting operation that takes sulfer, saltpete, and produces a 'bar' of it, and then another craft reaction that makes 'shot' as an ammo. Should it be part of the vanilla release? No.
Actually, I see the future taking something like Sean's ModBase and incorporating it into the core application. I was looking at the way the Raws were set up, and it looks like you could potentially set it up to where you got a radio button to select which goblins you wanted: (Evil, Normal, Monstrous, Gunpowder) and it would use those for world gen.
Anyway, the more effort that is put into the physics model now, the more quality mods we're going to see being vanilla DF. Think less Halo here, and more Half-Life... Better yet, imagine DF's Portal in which your dwarves don't get weapons, only traps and physics gadgets (pull the lever to launch the fire demon into the gobbos to set them on fire, then dump water on that to kill the demon.)