An object is valuable only because a person/group/society views it as valuable.
What i would like to see is a system where either the player or the society basically sets what the economy uses and how it functions, but is generally centered around the creatures performing necessary functions (eating, drinking, ownership) and using the economy to "pay" for those functions.
When the economy is activated, the dwarves in charge of the economy get together and form the economic rules based on the current fortress setup, personal preferences, beliefs, trade agreements, etc. Eventually depending on the fortress setup, they'll come to a conclusion about which items are worth what to who and when. If your fortress is heavilly geared towards metal work and there's enough of a metal that all the dwarves "agree" on what they like, that metal will be the economy material; but if the fortress is heavilly geared towards bone crafts, then creatures' bones and their totems might be the economy material/item.
Or the player could set it so that something completely arbitrary like [masterpeice dimple-cup dyed masterpeice giant cave spider silk cloth] is the (or more simply, just dyed with dimple cup dye, or studded with adamantine) is basis of the economy, and then just unpause it and watch it go from there.
Obviously workshops have to be enhanced to perform very specific actions to make sure the right symbol gets engraved on the correct item, but that would probably happen eventually anyways.
[edit - likewise, the player would get seiged by goblins because the goblins heard from someone that there's alot of material that the goblin leader likes that's inside of the fortress; of course, that doesn't stop those lone goblin/groups who like the other things from attacking -- the fortress value would be entirely dependent on what your dwarves and everyone else thinks of it. if they think nothing of it, your worth will litterally be zero, because that would be the fact of the situation.]