I make this sort of thing and liiterally throw in different things every single time. Some variations on what people have already mentioned might be a can of Campbell's tomato soup or mushroom or celery soup, Lipton's Beefy Onion soup mix, Worcestershire sauce, Montreal Steak Seasoning, a packet of McCormick slow cooker beef stew or pot roast mix, red pepper flakes........
I add all kinds of vegetables--celery, onions, carrots, peas, green beans, corn, lima beans, a can of tomatoes--whatever happens to be in the freezer or pantry.
I slow cook it in the oven (after giving away my crockpot that cooked too fast and too hot), and as long as the meat gets a chance to cook long enough to really tenderize and almost become shreddable, my husband loves it.