Some suggestions to help the flavor of the casserole would be garlic, onion, or Worcestershire sauce. Any of these would be great options to add to the ground beef while it is browning. This would add some amazing flavor to the meat without having to increase the prep time of the dish. Two garlic cloves, half an onion or a Tbsp of Worcestershire sauce would be plenty. Feel free to add any or all of these to give your dish a bit more flavor.