That is already the case for Stonehenge, which I hate. However don't forget that the Great Wall is built of locally available resources throughout its length. So stone in one place, brick in another, mud in another and sand and brush in yet another. I think the original intent of these bonuses was that if you had the bonus you could build the wonder in the same way as it is now but if not then you would still build a structure but not of the same material. Stonehenge has many similar examples, some built of stone which is not resource quality and some of wood.