The soup thickens up a bit after it cools down, so just add in some more beef broth before you serve it again and it will stretch even further! If you want to make the soup thicker add in a couple of tablespoons tomato paste.
You can also add in other veggies such as green beans or peas. Add these in with the corn.
To make the hamburger soup more hearty you can also add in some pasta such as macaroni noodles. Add in during the simmering time.