You need some type of grain in a meatloaf; yeah, it does stretch the meat a bit, but the real reason is that it (plus the eggs) act as a binder and hold the mixture together so it can be sliced /not just fall apart when it's cut.
Personally, I usually use a big handful of oatmeal, but if I have a bunch of bread going bad I'll run it through the food processor and make a bunch of crumbs. If I use bread crumbs, I often add milk to them -- makes the meat loaf a bit more moist. Stuffing mix'll work. I'm also quite partial to barley, but I'd only use it for half the grain in a meatloaf -- a little can go a long way. Really, any grain.