Today’s invention is a bicycle wheel with no rim, but, say, three or five spokes.
At the end of each spoke would be a further hub with a small wheel. These wheels would have different tyres, so that the rider could select one on each main wheel, according to the terrain.
If you wanted to ride up steps, the main hub would be unlocked, allowing the small wheels to move from step to step.
Passing over very soft or uneven ground, two small wheels could be used on each main wheel at the same time, with the hub sprung so as to allow a small clockwise and anticlockwise motion of the main wheel.