UPDATE:
BljdgnkirngilksjASJIFSLJKGNKJ GDSD OH GOD THE MATHS
Ahem.
There are some problems with how I wanted to do the tiles. Namely, rather than 32 tiles as I originally planned for, seamless digging without convoluted and unrealistic terrain smoothing requires upwards of 36 more tiles. This means more artistic work to create tilesets, more work to make the generators for the terrain, more work to make sure everything meshes absolutely smoothly (kinda), draws properly (probably), and translates to physics objects well (definitely).
In other words, a hell of a big headache that probably wouldn't look any better than if I just used a texture with alpha maps for tiles instead.
So that's what I decided to do, going with my original plan to have each tile use two bits per quadrant for how it's been mined. That is, before realizing that it still leaves a great deal of work in regards to terrain generation and translating the whole thing to polygons, not to mention making sure it snaps to surrounding terrain properly as it's dug away. As I mentioned in the first post, I have other things I'm working on, so I could either spend months getting these very basic mechanics ready, then several more months creating acceptable environment definitions to generate terrain, or I can reduce the complexity of the system.
If anyone thinks they can come up with an elegant solution for making the quad-based tiles work, I'm open to discussing it. Otherwise, I'm doing myself and future development in this a favor by reducing the tiles to being based on which sides have been mined instead.
This means that there will be sixteen tiles per tile page. Each side represents an isosceles right triangle out of the whole square.
(http://img.photobucket.com/albums/v99/Eagleon/newtiles.jpg)
It's sadly clunky, but it allows for actual textured graphics, which means there can be stuff done to make it less ugly hopefully. Since there are so few tiles, it'd be much easier work to create many variations in the terrain, and even transitions between different terrain types (something I was dreading). It also has the advantage of using only four bits(!) per tile, meaning I can use the other four bits left in the byte-per-tile to store other information.
And it will allow me to proceed with actually developing the damned thing :P
Other ponderings - Box2d seems to allow for ropes. This makes me dream of a piton gun; shoot one end into an anchor point, shoot the other into a physical object like a block of terrain or an enemy, and it'll swing around with appropriate force. If it's too heavy, it could yank the piton out at either end (if fired from too far away) or break the rope. Fun! Lots of possibilities there.
I was looking at mood rings earlier today and had an idea for this. I'll put it in spoilers because it sounds like an Easter-egg type thing.
Basically, to have mood crystals. I had ideas on three major things that could be done with the crystals in various forms (lenses, prisms, rings). First, being able to tell what your character's or an npc character's mood is. The different moods could each have both positive and negative effects, and could be based on what has recently happened to a character. Then they might also be used in a way to produce, for example, a beam with an affect based on your mood (angry=fire, happy=water?). Third, they could in one form be used to control your characters mood (but with something to restrict how often that is done). Plus, there could be other cool stuff they could do. Just some food for thought here though. If it sounds too complex or doesn't seem to fit game flavor, Earthsage is still an awesome idea without it.