Yup. I always found it weird that we had so many above-ground plants compared to underground plants. This is DWARF fortress with an emphasis on all things dwarfy. Then DF2014 hit and we gained an EXPLOSION of plants... yet not a single additional subterranean one.
I'd personally like to see more poisons added to the game. Not just extractable substances from fire snakes, but blight tubers and goblinbane or some other such that can be found deep in the caverns. After being extracted to a glass vial, marksdwarves could carry it on them to tip their bolts. Conversely, we could also grow medicinal herbs/plants "Hearty Moss". There is already golden salve (I know its technically useless, but it sounds like some type of medicine) on the surface. A plant that has sap that could double as an adhesive (to coat traps/through on the floor) could be found on the 2nd layer. Spices or tobacco-like plants could be grown as either trade goods or to provide happy thoughts. "Cavern kick" could be inedible by itself, but when milled into a powder could be added to an already prepared meal (like decorations are added to other goods), act like liquids by being unable to serve as a "base" but free to add to another food-stuff, or act as a final "5th" ingredient. However it is added, it would add a happy thought to the dwarf eating it even if he didn't prefer the meal "had an absolutely delicious meal recently!" The tobacco-like plants "Miner's delight" could make a happy though or just increase a dorf's stress tolerance. An incredibly fibrous/oily mushroom that could serve as a fuel-substitute... say "fire caps" 5 fire caps could serve as one charcoal/coke. I'd love to see some type of metal/stone based plant. It would be a nifty prize to snag out of the third cavern layer. Steel-stalks or razor bards could be a processed by a strand-extractor and serve as a poor man's candy. I don't want it overpowered, just as a substitute for a weapons-grade material if you struck out on your embark screen. I'm envisioning a fractional system like melting items into bars, where every 1 unit of "steel stalk/razor bard" would convert to 1/10 of a steel stalk wafer or something. The idea is to balance the infinite source of it with extra dwarf-power behind it.
We could just pointless add variety to below ground crops, creating 3-4 (10) virtually identical plants that are completely interchangeable to the point that many even grow within the same biomes (how many -milet plants alone do we have in addition to other flour-only plants?) While variety would be nice, empty variety is still empty. If nothing else plants of higher values should be lower in the cavern layers. Sweet pods and Quarry bushes are the two highest value plants in the game but can easily be snagged out of the first cavern layer. So sweet pods and quarry bushes could be moved to lower levels while more generic/low value crops could take their place alongside plump helmets in the first layer. Maybe something called "Elf-Necks" (i'm imagining a very daintly/spindly mushroom with a satisfying "snap" as you break it XD)