One good substitute for potatoes in beef soup or stew is turnips.
I've been making beef barley soup for years with turnips and never told mr. mm.,P> One day I didn't have the turnips and had to use potatoes and he noticed the difference and said something about the potatoes being all weird.
I said "That's probably because I used actual potatoes this time."
The pause in the conversation was hysterical. He isn't often left speechless but I got him that time.
I have also heard, but have never tried it, that mashed cauliflower with enough butter and salt makes a great substitute for mashed potatoes.
I have subbed parsnips....may have to try turnips.