I saw one recipe that said something like: throw whatever vegetables you have in a pot, but not root starches like potatoes.
She doesn't remember who she sent it do, does she?
I've got a book on stocks, broths and soups, I think has several classes of vegetable stocks, one major factor being whether or not there is tomato present. (That produces a dark stock, most other vegetables produce more of a white or clear stock.)
I've got celery, onion and carrot pulp in the freezer from my vinegar trials (the onion one was a failure), I plan to use them to make a stock when cooler weather is reliably here.