In a large skillet or dutch oven, combine ketchup, Dr. Pepper, brown sugar, apple cider vinegar, Worcestershire sauce, onion powder, and garlic powder. Season generously with Kosher salt. Bring to a boil over medium-high heat, then add meatballs and lower heat to maintain a simmer. Cook, stirring occasionally, until sauce has thickened to a glaze-like consistency, about 20 minutes.
Place four meatballs in each sub roll, top with cheddar cheese, and place on aluminum foil lined baking tray. Broil in oven until cheese is melted and rolls are toasted. Top with chopped avocado and pickled onions. Serve with extra Dr. Pepper BBQ Sauce. Devour immediately!