Nutmeg does interesting things to savoury foods. It's subtle though. It's often like a background singer making the other flavors better more than being a diva in its own right. I haven't added nutmeg to potatoes yet, but now I think I might try it. I usually add some fresh ground nutmeg to my spinach and bacon quiche. Nutmeg would also be right at home in a sweet potato pie. So why not mashed potatoes?
I've never used cinnamon on my potatoes, but I add it to some savory dishes like chill. The key is, cinnamon cuts through, so you need veeeeeeeery little.