Additional vegetables add flavor, color, as well as nutrition. They should be added to the soup in sequence according to how long it takes for them to cook. Your choice in vegetables will be guided by seasonal availability and your own tastes, of course.
Add starch when you want hearty filling soup, especially in winter. Diced potatoes or yams, pasta beads, crumbled dried bread, pieces of corn chips, tortilla strips, as well as an assortment of grains or beans all add weight, thickness, and flavor.