Have you considered adding growable trees/treehouses? In vanilla df elves live in actual trees that can be visited in adventure mode.
It could also be cool to have wild animals show up as visitors to your temples and such.
Using the plant plugin, I can allow the player to grow the local trees. That's a good idea indeed.
Wild animals as visitors should be posible, using the Nature Shrine to call a wildbeast and giving it the status of a resident or a mercenary (so you can add it to your military without being able to assing any job). It should work in theory.
Idea for druides http://www.bay12forums.com/smf/index.php?topic=173966.0
Second idea - using druide skill for making grown wood armor, weapons and crafts. Workshop will be designed as growing tree in vanilla forest retreat, but possibly smaller.
Penalties from killing animals may be realized by invisible gas, that add synrome only to elves. But I think this may work strange and you need find another way. Have you considered adding growable trees/treehouses? In vanilla df elves live in actual trees that can be visited in adventure mode.
It could also be cool to have wild animals show up as visitors to your temples and such.
Dfhack can grow tree manually by using special workshop.
The idea of a druid that turns their enemies into trees looks strange to me; imagine being playing as a goblin adventurer, traveling through an elven retreat and suddenly dying 'cause a random druid attacked you with his magic. In a very, VERY old version of Dwarf Fortress, elves carried a druid
(or more) with their army, and he was capable of turn your local trees into treants.
I already added the Grower's Garden, that uses the MAGIC_NATURE skill to grow free wooden items but those reactions can only be done in Spring (and will be canceled in any other season). A growing tree like the ones in the vanilla elven forests would be impossible to mod in.
I was thinking in using DFhack to track the death of any animal in the map, and if the killer was one of your elves then he or she would be marked as "hostile against wildlife" and thus being incapable of gaining the MAGIC_NATURE skill anymore.
My opinion - spellsongs, golden glass and fairy dragons isn't for dwarf fortress. Treants - possibly, but a lot kinds of magic is incongruous if add them to dwarf fortress.
While I'm not a fan of magic mods for DF -because they usually destroys the Legends Mode or only works with the dwarves of the player (and the magic is never applied in worldgen)- playing as the elves without magical stuff looks weird to me. In vanilla DF, they actually are a bit magical; animals never attacks them, trees are shaped at their will and grows the items they need to live, and are immortal.
I just want to give them a little of magic and spells so they're not like standard dwarves, but also not like warlocks. Since I don't like the idea of elves engaging trade (because they dislike it, and being able to turn upset the traders of your own civ looks weird), giving them the capability of protect themselves using spellsongs and druidism isn't unfair.
The idea of Golden Glass come to me after realizing that, without metals or gems, the elves would be forced to use only weak wooden weapons or (if you're lucky to find a volcano or a downward passage that gives you access to a magma pool) petrified versions of the same items. I was thinking in using amber as a weapon-grade material, but vanilla amber isn't pretty better than wood.
Another idea that could be great is adding a fake amber material that replaces the true amber in the reactions to forge weapons and armor, giving greater stats to the products than actual amber.
Treants will be simple: you produce treantish seeds using regular seeds and magic, plant them and wait until a treant sapling grows, then collect the plant and with a reaction you summon a child treant. The reactions to make the seeds and turning the sapling into an actual creature would not be cheap.