This thread made me hungry for a dog, so that’s what we’re having for dinner, along with some baked beans (Bush’s, doctored up). As for toppings: we have a jar of sauerkraut that we ordered from Katz’s Deli in NYC, along with some leftover Reuben relish. A bit of mustard to round it out.
For the record (again), ketchup is a plague upon civilization, unless used to make something non-ketchup-y like a glaze. You might as well dump a spoonful of sugar on your food, which is why my wife likes it. Mustard is the “normal” condiment for sausages, along with onions or pickle relish or kraut. As always, whatever you like is fine with me, but you risk being called ‘uncivilized’. 
My exception to this (and you really should try it), is crunchy peanut butter and sriracha. Think about it for a second: smoky dog, sweet peanut butter, and the zap of the pepper sauce. It’s a winner.