Hmmm, I assume the usual hamburger/hotdog isn't what you're looking for.
I think the least common thing I put ketchup on is kraft mac n cheese. Don't do it every time, but every now and then, when I'm having the standard, package only mac n cheese, I'll put a dab on there.
Can't use much, or it sucks. But the kind of amount you might drop as you're taking a bite of something else, that works great. That's how I found out I liked it, back as a kid.
Even though I'll incur the wrath of all Italians I do like to put ketchup on my pasta when I don't have the time/energy/ingredients to make a proper sauce. Even better with shredded cheese.
The night before I mix two eggs with a little milk and put it in a microwavable mug. Then microwave it for 30 seconds, stir and repeat twice. It creates a round slab of egg that I put on a piece of buttered toast with ketchup spread on it.
One time I made pasta for myself and a friend (also roommate). The sauce a bit spicy, with herbs, simmered for a while. One of my favorites. We sat down to eat and he said “yum this is so delicious.” Then he got up, walked to the fridge, grabbed the ketchup, and squeezed it all over the pasta and sauce.
i have so many spare sachets of ketchup from mcdonalds, and recently i've been experimenting with them to make a good sauce to mix into plain noodles. ketchup, sweet soy sauce, paprika, chili oil, chili flakes, toss in some random spices, and that's a good noodle sauce.
before finding that combination, i've gotten quite a few that were way less palatable.
my alternate answer is spaghetti, just to piss off the italians.
I used to mix it in with fried rice that'd been left sitting out for too long and turned really dry. Gives it some moisture and a vinegary edge, but probably not for everyone, since ketchup's trademark is stomping all over the subtle flavors of a dish.
When I was in elementary school, I'd dip my pizza crusts in ketchup at lunchtime. I still do that every now and then with Sriracha ketchup
Also, same elementary school lunch: on pizza days, they also used to give us a side of tricolor fusilli straight-up. Just plain pasta without even so much as a little olive oil. So, fuck it. It got blasted with 'chup.