What are good ingredients to put into a Scrapple soup stock?
I don't think it would work too well to just throw scrapple in a pot and make a soup. You'd be better off trying to make something "inspired" by it, trying to duplicate similar flavors in a soup. I'd try a thick pork soup, almost a stew, with chunks of pork shoulder and cornmeal-buckwheat dumplings, some sage and corriander and plenty of pepper. I'd definitely use a pork stock made from pork bones (and a pig's foot, sawn in half, for extra body), and ramp up the flavor with a ham hock. Maybe simmer some greens in there for the last few minutes to give it some color.

As for what to do with your existing loaf of scrapple, well, slice it, fry it up, and eat with fried eggs and ketchup for breakfast.