Maybe this is just coming from the fact that I've lived in the Sonoran desert for most of my life, but the fact that some deserts in this game are full of saguaro cacti and lack the other plant life native to the only regions where saguaro grow kind of bothers me. I understand that this is a fantasy game, but I find deserts to be supremely boring locations given their lack of interesting plant life that should otherwise be there. So here are a few of my suggestions, I'll try to break them up between categories to make reading this a little more palatable.
--Trees--
The Saguaro is perfect as is, I love it, granted it isn't a tree the game treats it as such and everything about it is so true to form that it sends my heart all aflutter. *Ahem* but the desert has other trees, namely the Mesquite tree.
Mesquite: Very dense wood, typically with only one very long taproot that goes down several hundred feet, shallower roots are typically much smaller. The tree could possibly be harvested for mesquite pods which are edible.
--Plants/shrubs/surface crops--
Now here I'm only going to really talk about two that would have any value in the dwarf fortress universe, mainly as a source of food and booze (We all know what gatherers are really for.)
The Prickly Pear: Probably the most common of the edible flora here in the desert. The Prickly pear has two main edible components. The pad itself (which would likely be considered a leaf for dwarf fortress purposes) and the fruit. They also have pink flowers that bloom but I'm unsure if these are edible. The fruit contains the seeds (which are poisonous, and thus inedible, even cooked.) and can be brewed into prickly pear wine.
The Agave: Agave is much more in depth when one considers it as it has been vitally important for native cultures within the area. So we'll take each piece of the plant step by step and isolate the uses of each one.
The leaves: The leaves of the Agave are very interesting to consider. The fibers of it are often used as thread and the internal part of the leaf is edible and can also be harvested for sap. The sap of the Agave plant can be fermented into
Pulqe, an alcoholic beverage that could keep your bearded midgets productive. I am omitting Tequila as a drink as that is named after the city of Tequila (and thus not a part of the DFverse) and only made from the blue Agave plant
The Agave also flowers once a year and then dies immediately after, however the large rosette stalk is often cooked and eaten, I'm unsure if it is edible raw however.
That's really all I can think of at the moment as I'm mostly going off of information I learned growing up, but I feel like the desert really needs a little extra something to make it unique.