Substitute ½ head green cabbage or bunch of spinach for the Swiss chard.
You can add one of the following to make the soup a bit heartier: can of white beans, ½ cup uncooked barley, orzo, lentils, or rice (while soup is simmering).
Use beef broth and add some cooked ground beef. Add ¼ teaspoon dried crushed red pepper with the oregano and thyme if you would like to add a bit of heat.
For vegetarian: Substitute vegetable broth for the chicken broth if you want it entirely vegetarian.
Leftovers, either cooked grains or vegetables can also be easily added to the soup.