So, we have these vast magma seas now, but aside from all that magma, and the occasional wandering creature, there's not all that much in them. So while thinking about shrubs and trees growing underwater, it hit me; why not have plants that will *only* grow in magma?
They should be more valuable than regular crops, to reflect the difficulty of harvesting them. They'll only grow wild within the magma sea, in squares of 4/7 to 7/7 magma. Once collected, in order to farm them, the farm must be placed on a muddy floor above 7/7 magma in order to feed the plants roots. Alternatively, the farm itself must be flooded with magma after planting, and drained in order to harvest. Plants farmed in this manner would not begin to wither until the magma was drained, to make farming them a bit more sane.
The magma plants themselves would mostly be processed to valuable food, booze and trade goods, but could also have unique properties. 'Magma reed cloth' could have a fixed temperature that would keep the wearer warm in even the most frigid environments. Magma tree wood would naturally have a magma-safe ignition point. There could be plants with strange extracts that burn and melt, or could be processed into unusual products, such as useable ash or even metal bars\gems.
And besides, what could be more dwarven than growing your food in magma?