Steam Shovels

In vanilla Minecraft, all mining must be done by hand, one block at a time, or by TNT, which destroys materials. In Buildcraft and Industrialcraft, the player may craft independent automated mining machines that take the player most of the way out of the mining process. The Steam Shovels mod (concept) extends the tech tree with heavy excavating machines that still operate under direct player control, allowing both a more engaged experience and better control of the outcome.

Tiers

 * Steam shovels: A modest step up from by-hand mining, about the size of a car and fueled by coal.
 * Backhoe: Familiar at any construction site, these medium-sized, highly mobile diggers are the workhorses of modern excavation. They work just like steam shovels, but they're significantly faster and run off of Buildcraft oil or Industrialcraft elecricity (via rechargeable batteries) instead of coal.
 * Draglines: Scaled-up versions of steam shovels drawing power from the local Industrialcraft electric grid, these machines commonly claim titles such as "heaviest land vehicle ever built" and "world's largest mobile earth-moving machine". They can move an incredible amount of earth in a brief time, at a cost of being very slow to maneuver.
 * Bucket wheel excavators: These modern monstrosities approach the size of the mountains they destroy. A large rotating wheel of scoopers bears down on earth and ore like a buzz saw.

New items and blocks

 * Machine shovel and arm: Crafted from a U-shape of iron (like a taller mine cart), this enormous scoop is the business end of any excavator. When it digs into a group of solid blocks, they are popped out into item mode, consolidated by type to reduce item counts and lag, and held within the shovel. The operator may right click to dump the shovel's contents on the ground.
 * Excavator tread: Like a grounded version of the Zeppelin mod, these wheels and loops of iron permit the construction of mobile vehicles out of other blocks. They are most useful when combined with other excavator equipment, which would otherwise remain stationary. The crafting recipe would be a full row of iron at both the top and bottom, with a row of redstone in the middle.
 * Operator cabin: When attached (directly or indirectly) to an excavator tread, the player may right click this block to control the tread's movements, as well as any attached machine shovels or bucket wheels. The crafting recipe is a shell of iron and glass surrounding a lever. Multiple cabins may be attached to the same excavator to permit independent multi-player operation of shovel and treads.
 * Bucket wheel: Crafted from a ring of eight machine shovels around an iron block, the bucket wheel is a far more efficient substitute for a machine shovel. Instead of accumulating blocks in a scooping motion for later dumping, the bucket wheel's rotation continuously removes solid blocks and pipes them through its arm to be excreted automatically behind the excavator.

Cost and scale
Since these machines tend toward the gigantic and expensive, we may want to replace some or all of the iron ingot inputs with iron blocks.

Or, since each tier would require similar items of varying size (they all have treads, shovels, and cabins), the choice between iron ingot and iron block could be used to determine the size of the object that is created, with an all-ingot recipe producing the rinky-dink little components of a steam shovel, intermediate sizes achieved by mixing blocks with ingots, and an all-block recipe yielding the full glory of the megamachines.

Construction limitations

 * Associating one shovel with multiple sets of treads doesn't make sense, but should it be possible to have one large set of treads with many small machine shovels?
 * If you want to use a shovel without treads, do you need an operator cabin, or can you right click the shovel directly?
 * How would players determine which cabin in a multi-cabin excavator controlled the treads and which controlled the shovel(s)?
 * Does the shovel/arm assemblage go all the way back to the base/anchor of the arm, or should the base be a separate piece? Should a larger arm require a larger area taken up by its base (ideally necessitating larger treads)?

Mobility
A dragline will move much more slowly than a person walking (Big Muskie had a top speed of 0.1 miles/hr, which admittedly would probably be excessively slow for this mod). This creates an unintentional incentive that instead of piloting them from site to site, the player would dismantle excavators, carry them to a new site, and reassemble them (likely forgoing treads entirely). Some means must be found to prevent this; perhaps it should be impossible to dismantle an excavator, or dismantling could destroy the parts. Or maybe some fraction of the original ingredients should drop in a disassembled state, as with boats; this would have the advantage of resembling the way that real draglines have been sold for scrap at the ends of their useful lives.

Similarly, we would have to decide how to handle changes in elevation, since the math involved in tilting these things properly would be very difficult. Real world draglines generally stuck to mostly flat ground (sometimes even reinforced with logs so their enormous weight didn't sink them into the dirt), so we have a good excuse for ducking out of the technical complexity in moving a large complicated structure over varying terrain. The best option is probably that a dragline, once built, can only move onto flat ground at the same elevation; if you try to move it up- or down-hill, it just stops, and you have to clear a path or fill in gaps (similar to Railcraft's tunnel bore). Maybe we could borrow the tunnel bore's strategy of having the vehicle fill gaps itself, and add a way to place blocks from the shovel below the treads' altitude instead of just dumping them.

This raises the question of what happens if you dig blocks out from under a dragline; I like the idea of playing some very loud creaking noises for 15 seconds before it explodes in a shower of iron ingots unless the player puts the block(s) back. But if people wire up synchronized charges of TNT under a dragline in order to remove all the blocks at once, it would look silly if it hovered there creaking and then finally exploded. Maybe removing a certain number of blocks should skip the creaking step and scrap the excavator immediately.

Zeppelin challenges
This mod will work best if the treads and shovels are usable separately but well-suited to being used together. The best model for treads being useful alone is for the player to be able to build a mobile structure out of normal solid blocks on top of treads, which strongly resembles the Zeppelin mod. However, the Zeppelin mod has limitations in both its stability and flexibility; interactive blocks do not function well on a zeppelin, but we would want it to be possible for a player to store supplies and process ore on board a moving dragline.

If the zeppelin approach cannot be made to work, we may have to forego the ability to put arbitrary blocks on treads; instead, an excavator would come to resemble more strongly the Planes mod, in which large but still unitary structures are built up from other parts and then placed as an interactive object all at once. This would reduce the mod's potential for creative use, but some could still be salvaged if it was possible to mix and match components of varying sizes, for example to put a tiny shovel on giant treads.

Spin-off concepts
If this mod can be completed, a number of similar ideas suggest themselves for further mods.
 * Cranes: Move big chunks of blocks from one spot to another with a big mechanical arm. Another derivative of the Zeppelin idea, but this time for the blocks being moved instead of the machine.
 * Bulldozers: Push many dirt blocks horizontally without turning them into items, like a giant piston.
 * Construction workers: New mob that spawns around heavy earth moving equipment. They move slowly but can give and take a lot of damage, and drop iron or better head armor.
 * The Walking City: so cabins & shovels are great and all, but the idea of building stuff on treads and moving it all en-mass is fantastic. A whole building on treads! Or even a collection of buildings! Wow!