Well, in the first Marble Madness, your controller (trackball) could only "thrust" along the X and Y planes. The marble's position on the Z plane (up and down) was only affected by the grade/level of the floor and gravity, right? Wouldn't that work even in a completely 3D world?
Keep in mind that most controllers are not suited very well to map to 3D space, either... If you truly want 6 "thrust" directions, it shouldn't be hard to add two more, but you may have to think about the physical controls.
Keep in mind that most controllers are not suited very well to map to 3D space, either... If you truly want 6 "thrust" directions, it shouldn't be hard to add two more, but you may have to think about the physical controls.