A three meter sphere, sitting in a matching cage (2/3 buried in the ground, or steel supports in a trailer) like a ball bearing cage. The thing could move all directions, and have some mechanised rollers onto the underside of the sphere to stop it spinning too quick or drive it slightly in certain directions of spin.
The user gets in the hatch from the sphere's (slightly weighted) upright position, standing with the wireless HMD powered by a small battery backpack w/ attached cushion for sitting down.
A pair of haptic gloves to simulate joystick handling, keyboard feel and scenario interaction.
I think that would be enough for a really good pair of gloves and HMD set to create a persistent immersion.
