Doh, I actually forgot stone in there. But apparently, nobody noticed, so maybe it never was as important as I thought.
Stone could be important for fortifications and road maintenance. Again, you could still build forts without it but it would cost you more, and you would still have roads, but to really have them in a splendid condition, stone is needed.
Though I really would like each resource to be tied to the region type for simplicity's sake. Just by glancing at the map you would get a general idea of what is produced where, or what could be produced. Stone and metal would both naturally go to the mountains. I do not know if this is a problem, perhaps it is not, but it would be nice to have just one resource / region.
