Any kind of beans - haricot, pinto, butter, bortolli, kidney, etc, will give it body as will chick peas.
If the flavour is a bit weak, a splash of wine, sherry, Worcester or soy sauce will 'taste' it up a bit as will a pinch of herbs and Knorr stock cubes - especially the chicken, ham and vegetable types will add flavour rather than trying to add too much salt/pepper to try and give the soup more 'depth'.
Love the idea of the Parmesan cheese crust as well!
Adding pasta is always a great option if its a bit 'thin' and if you really want to go to town you could make some dumplings - they make a soup into a real meal.
You can do all sorts of stuff with them - herby, herb and garlic, cheese. cheese topped, spicy, veggie, curry, leek, onion - you can really use your imagination to suit the occasion!