Personally, at this very moment, I'm modding in plants that only appear on the second and third cavern layers. The dwarven civilization (as far as I know) only starts off with layer 1 stuff, so all that I mod in is completely up to an intrepid player to discover and use. I like this feeling of pioneering, having your fortress be the first place to... do what I said.
That's another reason for which I'm recommending what I'm recommending. Give elves new stuff - stuff that a player coming from dwarves wouldn't recognize, instead of the same old deep-cavern things that both aren't in character and new. That's the point of mods, if you ask me! I don't know if you do.
Since your heartwood workshop thing can generate wood from nowhere, you could even make wood that doesn't normally exist in nature. Go wild with that idea! Give the wood various properties - being very cold, very hot, being tough, very light, contact syndrome (not poison oak), whatever you want, really.
One other idea you could have is similarly have another workshop generate seeds for you - seeds not necessarily found in nature, or found rarely. Conjure up sunberry seeds, valley herb seeds, and whatever you can think of.
Also, if you are looking for ideas... I think 'Conservatory' would be a nice name for the woodsinging workshop, split off from the basic craftself one. Might want to disable the hardcoded 'make wood items' workshops and replace them with that.