That's a great idea with the instant potato flakes!
I've also used Masa Harina to thicken up chili's and soups in the past. It does have a rather strong corn taste though so I wouldn't use it in everything. And what I do use it in, I use it sparingly.
Generally speaking, any soup with beans in it will thicken up quite a bit on it's own as long as you cook it for a long enough time. I make a black bean soup that looks watery at first, but after a couple hours of simmering it is super thick without having to add any extra thickeners.
Corn starch you need to be careful with. Always use a slurry with it, don't add it directly to the soup because it will lump up on you. It tends to create a translucent, sort of slimy quality to the soup that I'm not always crazy about personally. A properly cooked flour and butter roux will work better in most cases imo, but I've been told the more you darken it, the less thickening power it has (although the more flavor it has as well).