Root cuttings are one of the easiest ways to develop a good bonsai from throw away materials. Repotting a fig bonsai is needed every year to 5 years depending upon growth, species, size of pot etc.
On removing some of the larger and more shapely roots the root is potted up with the root hairs in the soil and the cut end sticking out of the soil about 1-2 inches. After 2-6 months the end of the root will sprout new leaves and stems. Over time the new growth can be shaped to be an interesting bonsai. In addition most of the root can be lifted out of the pot to make a longer trunk.
Not all fig species will sprout from roots but natalensis is one that sprouts very easily.