Try other deli meats like smoked turkey or thin slices of roast beef. Pickled red onions would also be quite tasty. I like a little mayonnaise on my toasties, and chilli sauce is also very good. (Like my homemade Chilli Sauce or Ballymaloe Sauce.) Just remember: don't get too fancy (it's just a toastie) or use too many fillings that will end up leaking out of your hot sandwich.