Oh definitely onions and carrots. The potato might be nice, but that's also a starch, so I don't know if I'd mix a potato with noodles, but don't go by me.
Well, actually, the point behind the one potato isn't so much for eating... the potato, when cut up small so that you have the pieces floating in the broth/mixture, it does a good job of soaking up the extra salt that the soy sauce added. That leaves you with the flavor of the soy sauce, but not the salt from it. Just don't eat the potato pieces! I pull the potato pieces out and put them aside for the compost before serving.