Pourquoi pas une autre chose?
"Autre chose" is a set phrase.
Why not voulez-vous?
Why not "d'autre chose"
You could have "d'autres choses" as the plural of "une autre chose", where "d'" would stand for "des" before an adjective.
Yet "something else" is "autre chose" or "quelque chose d'autre".
I have heard "...d'autre chose" used in a French restaurant.
You can hear that if the verb is not "manger" but, for instance "avoir besoin de": "Avez-vous besoin d'autre chose ?"
Tu voudras....marked wrong. Don't understand why unless I am missing something