It's not something for me now that I'm older but I use to love putting ketchup on mashed potatoes and mac and cheese. I know mac and cheese with it is somewhat common.
Also potatoes on a hamburger is really good. If you do a cheese burger it might take some trial and error on getting the right cheese. I myself like fresh shredded American. Don't use cheap ingredients though! This is the 1 thing I wouldn't cheap out on.