Parsnips really are the star of this soup, and I wouldn't recommend adding anything else because it would totally change the flavour. You could do a mix of carrots and parsnips though if you wanted something a little different. We have used shallots, however you could swap those for onion or leek. Most soups use potatoes to give it some thickness, but I don't find that this soup needs that, as the parsnips make it really thick anyway.