I would prefer for elves to be actively phytokinetic... they can force trees and other plants to grow any way they damn well please.
Yes, I was just about to start a thread about what it might be like, in some distant-future version, to play the elves. But this suggestion is exactly what I was thinking. I searched and found this thread, so I'll post here instead of starting a new one.
I think it would need to start with multi-z-level trees made of a new class of material:
living wood. Then you'd have a profession of "tree trainer" or something. You'd designate squares for:
- Grow wall
- Grow floor
- Grow ladder/stairway
- etc..
And if there were adjacent living wood, a tree trainer would go to that square, do something with the plant, and wood would grow into that region to create the needed structure. A bit like a dwarven mason, but slower and requiring no materials.
To get lumber, another specialist more like a dwarven farmer would induce trees to grow extra branches that spontaneously fall off, producing logs without killing the tree. The logs would fall to the ground (keep your elves out of the way!) and haulers would have to collect them.
Elves could cut trees if necessary, but doing so would generate severely unhappy thoughts when doing so, maybe in the -30 range. Even the elves not doing the cutting would get (smaller) unhappy thoughts.
Elves could still have masonry and dig stone, but it would be much slower process than for dwarves. They could not go underground - the reverse of dark adaptation, but it would likely be fatal. So mines would either have to be surface/strip mines, or stone could be purchased from dwarven merchants. If you're in a forest and have to cut trees to do it, well, then that just sucks for you.
But you could use the little stone you have for making walls around the trunks of your city, making them harder to set on fire.
Defending a fortress would consist of climbing up the trees ("order: Elves stay off the ground") and pulling up the rope ladders. (You'd start with ladders of living wood grown up the trunks, but remove them once you had a cloth/fiber industry and use rope ladders the way dwarves use drawbridges.) Since goblins or dwarves might try to chop at the trees or even set them on fire, plenty of archers would be necessary - elves can't simply close the door and ride out sieges the same way.
You could also dig moats around your trees, or even the whole forest.
Farming could include regular above-ground farming, just like dwarves. In addition there would be tree-farming, which could only occur on plots of living wood, in which arboreal farmers induce branches to grow fruit. This would give elves a new option to parallel dwarves' underground farming.
Rooms probably wouldn't usually have walls, instead they'd generally be branch-floors surrounded by empty space. Hmm, that could make sparring hazardous. "Fimbreth, champion wrestler, dodges strike. Fimbreth falls to his death!" :-)