just set it up so that building can be built, and npcs hired as shopkeepers, and I myself will set up a trading post for the miners.

Your idea with layers and cubes is pretty confining: why not just do dynamic terrain in full 3-d? and with a level collapsing, wouldn\'t only the part with an excess of tunnels collapse? it would be cooler to do a realistic simulation of how the tunnels would fall, and run it so that everything fell realistically. of course that is probably processor intesive, so a fall would only need to be simulated about once a day or when there is a nearby explosion (note that in a real mine, collapse usually doesn\'t occour immidiatly, it usually requires a shock to trigger it. Also, collapses generally produce enough of a shock to trigger other collapses, so it would not be unrealistic for them to occour all at once. I doubt, though, that the devs are going to do this, because it will likely be quite a bit of trouble to integrate an engineering library into planeshift, even with such great, motivated devs.
Terrain should be dynamic, so that anyone with appropriate skills and equipment can just start digging a new tunnel someplace, whther there is ore there or not. The other application of this would be building forts out of dirt, hollowing out caves for houses, etc. Mining skill would be used primarily to locate veins of ore, so you know where to dig. Mining skill would also increase your speed of diggingand cutting through rock. of course, selling mining permits would be entirley up to the person who own the land the mine is on, and thier ability to kick violating players off of it. using rent-a-cops

Everything you said, tbenny, would happen automatically, given some very generic rules about the planeshift world.(own territory, veins of minerals, which can be exausted, dynamic terrain, hirable npcs, ability to build buildings)
None of this stuff is exactly easy to implement, and I wish good luck to devs, who have come so far, but still have far to go