The problem in my mind is that the economic system of the game is just completely and totally broken.
This is true.
First, stone crafts just need to go away. This isn't the Stone Age. Your five hundred drinking cups carved from only the finest chalk have a market value of zero. The few stones pretty enough to be made into valuable crafts are called "gems".
Second, caravans should be far more discriminating. Not articles of trade: ill-fitting armor that smells like dead goblin; fish stew; stone mechanisms*; anything below +item+ quality. Caravans come to your fortress expecting to buy the fine products of dwarven artisans, not Aunt Betty's collection of slightly chipped shotglasses and antique beef jerky. Merchants should get offended and storm out if you try to unload your trash on them.
Third, as mentioned before, farm production is totally excessive. So is animal butchery. So are eggs. In general food is not enough of a problem: there's hardly any labor input (crops require no labor other than planting and harvesting, and animals never have to interact with a dwarf other than the butcher!) and no capital input (which is part of the poor pacing of the game--you don't incrementally build up your farming industry over time because there's nothing to build).
it's fine to say add stuff to the caverns that players would want, but can you think of anything players would want (that's not weapon/armor material since adamantine has that covered)?
That's what we're thinking of with the requirement for crops to be grown in the caverns: what the player wants is farmland, so make it available only in the cavern. This is nice because (unlike adamantine) you can't just dig it out and then close up the hole--you have to keep control of the territory.
Before the caverns get overhauled to be more worthwhile, the whole economy of the DF world needs to be overhauled.
My disagreement here is with the
before. There's no reason "the whole economy" needs to be fixed first. Even
without fixing the economy, forcing underground farms to be in the caverns would be a great improvement for a very modest amount of effort. It would be an even greater improvement if farms were actually necessary for survival, and better still if they were necessary for trade. But one thing at a time.
* stone mechanisms also need to go away