incorporeal movement

The problem: Incorporeal creatures are not subject to frictional forces from physical objects, and therefore cannot ride in vehicles or other transportation vessels without having to consciously match speed. This makes it impossible for incorporeal creatures to travel in carriages, cars, subway trains, airplanes, spaceships, etc., and makes transportation to other other celestial bodies such as other planets, natural and artificial satellites, space stations, asteroids, etc., almost completely impossible. The planet as a permanent frame of reference is completely arbitrary and severely limits the potential for effective use of incorporeal creatures in any given campaign setting.

Sudden change in tone:
Imagine a futuristic space/horror game in which the player characters need to abandon ship and glide to an abandoned satellite from which they send a distress beacon and sit back and wait for rescue... only to find a few hours into their chillaxing that the space station has a wraith on board, which was responsible for the deaths of the satellite's previous crew. The PC's have one haven that the wraith cannot enter, in which they're safe but the conditions are cramped and uncomfortable, but they need to venture out every now and again in order to keep the satellite from exploding.
How frickin' badass is that? It'd make a great story, wouldn't it?
But think about what would happen if an incorporeal creature started to fly up from the surface of a planet. Incorporeal creatures are completely unaffected by gravity, so the frame of reference that the creature adopts has nothing to do with how close to the center of the planet he is, so he continues moving at an equal angular velocity with respect to the center of the planet. This would mean that if the creature tried to enter any satellite that wasn't both geosynchronous and perfectly parallel to the planet's equator, he'd have to be consciously moving through space in order to stay inside the satellite at all. With any other satellite (which commonly move at several kilometers per second around a planet, a speed that no incorporeal creature can match), the creature's best chance is to get in the way of its trajectory and have it collide with him. That way, the incorporeal creature can technically be *in* the satellite, albeit for a fraction of a second, in the same way that a ghost's best chance to be *in* a train is to stand on the tracks and have the train pass through him, because there's no way the ghost can keep up by flying. And forget about moving with space ships - the creature would be left behind every time it moved, because he can't be pulled along by frictional forces from the floors or normal forces from the walls.
This is just dumb. Incorporeal creatures should be able to ride in vehicles and haunt creepy space stations without having to consciously move just to stay in the same place. Think of what this means for the ghostly crew of a haunted pirate ship - the crew would get drunk, and half of them would end up floating above the waves miles away from their ship simply because they forgot to be constantly flying at full speed in the ship's direction.

You could just arbitrarily decide that an incorporeal creature stays in whatever inertial frame of reference makes the game flow right, but I thought of what I think is a better idea that also allows for more creative ideas to build off of it.

Here's the idea:
Incorporeal creatures must "hook" themselves onto some mass and they move with respect to the head of an arbitrary vector coming from that mass. The creature automatically moves to preserve a constant displacement vector from the head of his hook vector, and his own movement adjusts his displacement vector from the head of his hook vector.
By default, newly created non-corporeal creatures hook onto the surface they were standing on when they die, with an arbitrary orientation (a vector pointing from the center of the planet to the surface of the spot at which they became non-corporeal). This has the effect of making them move with the natural rotation of the planet, just as we corporeal creatures do. A non-corporeal creature could spend its entire existence not even knowing that it has a hook vector in an arbitrary place somewhere on the planet, and it would never matter if that creature never left the planet; the creature would experience the planet's rotation just as any corporeal creature.

I'm not sure if I want to make it a feat or just a technique that you pick up after you've been incorporeal for a while, but imagine if incorporeal creatures could pick a new hook vector at will. This would mean primarily that incorporeal creatures can now move like a corporeal creature within a vehicle. When a corporeal creature is standing, for example, on a subway car, the car itself is pushing the creature along with it so that the creature doesn't have to move at all to stay in the same place, given the subway car as the inertial frame of reference. But an incorporeal creature, because it can't be pulled along by frictional forces from the subway car's floor, has to move as though it was keeping up alongside the subway car just to stay in the same spot in the car as a corporeal creature would. But if the creature simply chooses a new hook vector from the back of a car to the front, or any rigid mass that it doesn't expect will move or rotate indepedently of the frame of reference (picking a wheel, for example, would be a terrible idea), then it moves freely along with the car without exerting any effort whatsoever, and it can take its full movement *with respect to the car* just as any corporeal creature would.
(If the wraith was dumb enough to pick a vector along the axle between wheels, or from the center of a wheel to the rim, then its movement would be in an extremely rapid circular fashion, sending it face-first through the floor, again face-first through the dirt, upside down in an arc underground, face-first again above ground and through the underside of the car, and finally back to floating where it was in the subway car, and it'd make one such revolution several dozen times per second. This experience would most likely be extremely disorienting, and the creature would probably have to pick a completely random vector inside the planet to stabilize itself and recollect its senses before trying a different hook vector in the car, if the train isn't out of visual range by then.)

This implies a number of additional things that I think are extremely awesome, and can totally be used to make combat with incorporeal creatures that much more interesting:
1) If a non-corporeal creature picks a hook vector inside you while your back is turned, it can always remain behind you if it doesn't move to its sides. So if it puts a hook vector from the base of your skull to the top of your head and then stands still, then it will always stay out of your sight. This also means that you send it flying every time you turn your head, and if you're clever you can use this to your advantage.
Let's say your party is fighting a wraith who for some reason decided to put a hook vector from the base of your skull to the top of your head. You quickly look to your left to see what the wizard is doing and you notice that the wraith stays perfectly within your field of vision when you rotated your head to look at the wizard and then back again. To test that the hook is where you think it is, you nod your head and confirm that the wraith moves up and down to match your head movement. You shout for the wizard to make a wall of force somewhere, and then you snap your head quickly to face into the wall of force, slamming the wraith into it at a tremendous speed. If the wraith didn't die from the impact, it'd best pick a new hook vector on its action unless it wants to be pile-drived into the wall of force again.
2) If an incorporeal creature picks a hook vector inside YOU, then it will ALWAYS be able to outrun you, unless you teleport or plane shift or something. Under the definition of incorporeal movement, which is simply the adjustment of the distance between itself and the head of its hook vector, then that means that the wraith is ALWAYS moving at least as fast as you are, whether it wants to or not, and on top of this it can reduce the distance between you and it by its own speed. So if it can move at 60 ft per round, then the creature can gain 60 feet on you each round no matter how quickly you start running or flying away.
This can make wraiths with the ability to pick new hooks terrifyingly effective predators, and changes the PCs' escape strategy to "what will get me to a safe place faster" rather than "what will get me away faster," which is a very important distinction.
3) Similar to the previous, an incorporeal creature can stay away from the target of its hook. If a spellcasting ghost picked a barbarian as its hook and doesn't move, then the distance between them will never change. No matter how quickly the barbarian moves or how far, the ghost will always be the same distance away, which would force the barbarian to either use ranged weapons to attack the ghost or to back up so that his allies can attack it (and if the barbarian's speed plus his allies speed is greater than the ghost's speed, then the barbarian's allies can catch up to the ghost). But this also means that the ghost can choose to always be within melee range of a target. If a wraith decides to take out the spellcaster in a group of adventurers, he can remain right next to her no matter how far she runs or flies away. Then again, this is less useful because she could easily just stay within melee range of her allies and they'd help her out.
4) An incorporeal creature can theoretically move at or faster than the speed of light, particularly because it has no mass. If it finds an object that's spinning very, very quickly, such as a computer microprocessor, and has the precision to place a hook vector from its center to its rim and then moves some distance away, then its translational velocity can easily be faster than the speed of light.
For example, if a wraith picks a hook vector in a Pentium III chip that spins at 1.7 GHz (i.e. makes 1.7*10^9 rotations/second = makes a full rotation every 1/(1.7*10^9) seconds), then it'd only have to float 23.7 centimeters away from the processor to be moving at the speed of light (circumference * frequency is its translational velocity; circumference = distance/rotation, frequency = rotations/second, so circumference * frequency = distance/second). This would obviously be extremely disorienting, and you could apply Einstein's relativity to explain how the wraith experiences time.
5) An incorporeal creature might choose to drop its hook altogether and make its movement absolute. (I'm so far undecided on whether I want to make this happen if the object the creature hooks to changes planes.) We corporeal creatures are always being pushed or pulled by things and we don't notice; the Earth is in fact spinning very, very quickly, and hurtling through space at a ridiculous speed. We can't ever tell what in the universe is ABSOLUTELY still, only what is motionless relative to however we're already moving. So incorporeal creatures have a second truly unique experience available to them (the first being travelling at / faster than the speed of light), which is to experience ABSOLUTE stillness.
This would provide an instant escape plan for any non-corporeal creature. Simply drop your hook, and you're tens of thousands of kilometers away in a fraction of a second (just hope that you don't crash into any walls of force before hitting outer space).
Then again, this would pretty much be a great way to permanently lose yourself in deep space. If you were to suddenly get an absolute velocity of zero, anyone watching you would see you suddenly rocket off in one direction, and within a fraction of a second you'd be floating in deep space without any clue as to how to get back, since the planet you were standing on moments ago just flew away from you at 38,000 km/s following a crazy trajectory that's decided by 230 different variables. The heat death of the universe would happen long before you found any other conscious entity anywhere.
Unless you can plane shift or teleport or something, cuz in that case you'd be fine.

But really, most of these extreme situations wouldn't be used, and corporeal things likely wouldn't have a clue about these inertial hooks. Living things would simply acknowledge that ghosts can ride in vehicles as easily as a living thing and that you just can't outrun a wraith. Witnessing an incorporeal creature screaming through deep space at 4000 times the speed of light would definitely not be a common occurance, and only ghosts with harmless mischief in mind would most commonly put their inertial hooks on anything other than the ground.

-cvp