My family used to own a campground, and we used old semi truck rims as fire pits. Probably still have 20-25 of em'.
My concern with a mobile firepit would be with...how mobile? Just something you can drag around, or are you considering wheels, or skids? The heat from the fire might render any complicated movable linkage or parts inoperative in a short order. A fixed set of skids would seem to me to be the most pratical, BUT.. anything that projects beyond the diameter of the pit itself becomes a safety hazard when approaching the pit to add wood, while cooking, etc.
If you don't want to just drag it, maybe a trailer, or even a custom, removable dolly, or transport, to allow easier movement might work. It has been my experience that projects such as this, while allowing ample freedom to express one's creativity, sometimes involve more time, money, and sweat than is justified by the end result.
Keep it simple.
