Ooh, this is one of the best parts! The sky is the limit. Stick with classic ketchup, dijon, yellow mustard, or tartar sauce. Maybe you have a thicker dressing you'd like to use, like Thousand Islands or Ranch. For a twist, mix mayo and ketchup, or for a little heat, mayo and sriracha. Even a simple lemon squirt or a garlic butter drizzle would be great!