I recently saw this video about the Virtusphere, which I thought was very impressive, and got me thinking about it in depth.
I got an idea to apply braking or rotational force to the sphere to manipulate the position of the user. With this capability, you can create the sensation of walking up or down an incline of varying degree.
To clarify: When using the Virtusphere with no forces applied, the user will settle at the bottom of the sphere, where it is most level to the ground. But, if we apply a brake to the sphere, and the user continues to walk, they will begin to walk "up" the sphere, and the surface upon which they are standing is no longer level with the ground. By varying the braking force, we can manipulate the user's position in the sphere, which correlates to an angle of incline.
To create a downward incline, the sphere would accelerate in the direction of the user's motion until the user is behind the center of the sphere, and the angle of incline is negative.
Furthermore, you can simulate walking along a hill which slopes off to the left or right, or a spiral staircase(spiral ramp?).
While I don't have immediate plans to build my own Virtusphere, I think it's achievable, and incorporating these kinds of techniques greatly increases the versatility of the device.