I actually came up with an idea that I rather like: Simple Machines.
There are five or six simple machines from history which were described in the Classical age and commonly referred to in the Renaissance (depending on what account you use).
Archimedes described the first three around the third century BC:
Heron of Alexandria adds two more in the first century AD:
- Windlass (aka Wheel & Axle)
Finally, in the Renaissance, a Flemish engineer added one more:
- Inclined plane (added later)
Appropriate Tokens: I am not aware of any game which uses these items as tokens, or any set of tokens that would work right off the bat. There are a few solutions. Obviously you could make a set of cards or just have a list with the current inventory checked. I like tangible items for the subsystem, however. Hands on tokens also allow players to imagine possibilities more easily. So, instead I think I would make a trip to a hardware or craft store and pick up a few items.
- Lever: You need a board and a fulcrum for this.
- Pulley: We need, at a minimum, a rope/belt and at least one axle. A spool (as for thread) and a string or rubber band should work just fine.
- Screw: Easy. Get a screw.
- Windlass or Wheel and Axle: A spool with a dowel or straw through it, or a bolt and flat washer type set up.
- Wedge: A small chisel should be easy enough to find.
- Inclined Plane: Also fairly easy. Any triangular piece of wood, or a flat piece of wood that can be placed at angles.
I like the idea of these players actually having a literal bag of tools. Put all of these bits of "junk" in a dice bag/crown whiskey bag/mini tool chest. Dump them out on the table. It'd be awesome! It also lets everyone at the table visualize what is being done with the machines in non-standard applications.
Not Robbing Other Players: One issue I have with the Thief in AD&D is that once you add the thief class, other players feel like they can't do things like search for traps (even via roleplay) as it is a "thief function." Likewise, I do not want players to imagine that they can't do things like swing from a chandelier (basically a pully) or slide down a banister (inclined plane) or raise the anchor on a boat (windlass) just because they are from the wrong power source.
To mitigate that I would let folks with this power source do some unique and more powerful things.
- "McGyver:" Let the "mind" power characters use these simple machines even if reduced to improvised materials or no materials. There is the extreme case of being stripped naked and thrown in a cell. However, if I'd let a spirit character cast a playing card spell, or if a body character could invoke an aura or find a follower, then McGyver should be able to put together a simple machine out of chewing gum and bailing wire. In such an extreme case the McGyver just finds overlooked materials that others would not or could not imagine being used.
- Speed: Let the "mind" characters construct and/or use these machines orders of magnitude more quickly. Given enough time, anyone can assemble together a windlass that allows heavy loads (like adventurers or treasures) to be safely lowered up and down a sheer vertical mineshaft. But only McGyver can do so in the midst of combat. Each machine should probably have some sort of "instant" combat application as well.
- Quality: Anyone with common sense can probably put together a simple machine with a little mechanical advantage. For example, a simple one rope/one axle pulley has a mechanical advantage of one (it just lets you change the direction of a force). A somewhat more complex gun tackle has a mechanical advantage of two, basically doubling the work that can be done. A complex threefold purchase pulley has a mechanical advantage of six. Any Joe Schmoe can put together a machine with low mechanical advantage, but maybe it takes a specialist to get mechanical advantages of four, five, six, or better.
- Chance of Failure: A jerry-rigged device probably should have some sort of (substantial) chance of failure. Maybe McGyver's machines are fool-proof!
Ultimately it is probably the ability to be exceptional at all of the above: being able to quickly throw together a machine which works very well, is very reliable, using improvised or no special materials. Such ingenuity would really be close to magical and would represent extremely creative human genius getting work done in the physical world. It doesn't mean that other characters can't take time to improvise a lower quality machine, or slap together a shoddy piece of junk in a hurry, or spend time and money (materials) to "get it right."
Combat-specific applications: In a fantasy adventure game with life-or-death stakes, these special powers need to have a specific combat application as opposed to vague sorts of effects which can be discussed/negotiated out of combat. I also like the idea of a combat trap or terrain modification. The mechanical effect of the "combat application" or buff should be somewhat less than the effect of the other power sources. However, the terrain effect should be potent. Furthermore, if the terrain effect/trap is not triggered, then it should be recoverable.
Purpose: The point of a lever is to multiply force. I push down with 15 lbs of force, 30, 45, 60, or 150 pounds go up.
Combat Application: A straightforward bonus to damage seems appropriate. Do to understanding the masterful application of leverage, the player optimizes their weapon usage to maximize the force of their blows. Forced movement might also work: I step to the left three feet and throw a foe three yards away.
Combat Terrain: Simple damage dealing trap, using mechanical leverage to magnify the impact of a relatively small object. This is the principle that some snares for small game work on. Some other folks have written about levers as a trigger device for many traps. Alternatively, could be used as a super crowbar type thing to break open barriers/doors.
Purpose: The basic point of a pulley is to redirect force. I pull sideways and something goes up. At a more complex level, block and tackle systems multiply that force AND redirect it.
Combat Application: This is a sort of mechanical jiu-jitsu. Allow the character to redirect incoming blows and avoid damage.
Combat Terrain: The classic net trap! When triggered, everyone in the net is scooped up and into the air (or in another direction, depending on where the pulley is).
Purpose: Screws convert rotational force to linear force. Screws have often been used to fasten items together.
Combat Application: Screws prevent things from moving. So perhaps the player can "fasten" their character to anything else: they can lock their feet to the floor and prevent being swept away by an avalanche, pin themselves to a foe and stay locked in mortal combat, etc.
Combat Terrain: Attach two objects together. Or, a trap, that when triggered, sticks the victim to that spot.
- WINDLASS/WHEEL AND AXLE
Purpose: A windlass transfers or focuses force, generally with a lot of motion on the outside wheel and a lot of force on the smaller inside axle.
Combat Application: Sacrifice movement for attack bonuses. Or, sacrifice attacks for movement bonuses.
Combat Terrain: Wheels can be used for timing or resetting a mechanism. For timing, imagine a gear that clicks along until a trap is triggered or reset. For resetting, imagine a wheel turned by water, gravity (say, a heavy weight on a rope pulling it around). So, I would let this be used in conjunction with another mechanism/terrain to both time and reset it for multiple uses once triggered. I.E. as a "complex machine" featuring multiple simple machines.
Purpose: Wedges convert force in one direction to force in perpendicular directions. Wedges are often used for splitting objects (axes) or stopping them (door stops).
Combat Application: Gain bonus to armor penetration.
Combat Terrain : Super doorstop. Place to lock a door solidly in place.
- INCLINED PLANE
Purpose: Inclined planes raise and lower loads by spreading out work over distance and/or time.
Combat Application: I like the idea of spreading out some sort of unpleasant consequence or task over time. For example, delay taking a hit or paying some sort of cost. Another option might be to let someone take a double turn but then run a risk of losing future turn(s).
Combat Terrain: Make difficult terrain easier to pass over. Put a slide over a steep drop off or slope, allowing rapid but safe descent.