I think the elves need to play a more active role in dwarf fortress. This feature would introduce a naturally forming mega structure that interacts with the events in a fortress. The elves would come and plant a sapling in your fortress, a special sapping that the they had bred specifically for your fortress's biome. This sapling would grow into a giant "Earth Tree" or "Gaia tree". It would be presented as a present from their civilization, but you would not have the option to refuse it.
Formation:
-Elven diplomat comes when reputation is sufficiently high (Very few fell tree jobs, profitable trade between your civilization and theirs)
-Diplomat plants tree in suitable location and makes a special announcement
-Tree should start out as a normal sapling
-Growth should be rapid at first, slowing exponentially until it is mature after about 50? years
-The tree and it's root system should grow around and over buildings and items, destroying anything in it's way.
Features:
-Logs should fall below the tree during wind/rain/snow storms. The giant branches would be the size of normal tree trunks (thus explaining how elves have access to wooden items without killing trees). Logs should begin falling after 30? years.
-If the tree dies for any reason then elves will declare war
-Advanced forest retreats past a certain age should have giant trees with elves living in them.
http://www.bay12forums.com/smf/eternal_voting.php#vote109