Someone mentioned this.. briefly in an another thread, but I wanted to expand on it without hijacking..
So the idea is that you could play the very early Civs, back 1000 years before the dwarves, kinda give the game some BACK ground buildings, before the world was created..
When the game world gets updated, the Pre-Civs buildings/things/ etc are put through an earthquake (split into sections and moved) or are lost to the sands of time (buried under a few layers) Taken over by nature (maybe raised a few layers and then overgrown with trees and weeds)
Only Great Stone monuments, and multi layered walls would survive, Some specialised traps might still work? (stone fall/pit traps) maybe spikes would work once.. but never again..
As to WHO? and WHY?
The Concept is two fold..
1) you add more depth to the game with things you can discover with adventure AND dwarf mode..
2) Its actually good training for some of the basics, before you start your first fortress
E.g. Maybe Cavemen.. no tech, living in caves, hunting, fishing, cooking over fires, and fighting with pre-historic goblins, creatures that existed by the hundreds but now are just legends? - Trains you in simple task designation for your clan members.
Eldar Races, God like, Magic using, Super powerful.. People that can nominate the land to be dug, raised, dropped, built, smoothed.. all the activities that are available to the dwarves, but done Instantly, at the cost of magic.. when the magic is gone, the race dies.. The purpose to understand construction sites, create great looking halls and chambers, maybe traps, store goods and treasures found.. But for the race? the great magics of the world are dying and they must search for special rocks deep within the earth, and store them in special chambers to give themselves back some magic. Living as long as possible is the purpose of the builder..
Communities; In Early games, maybe a low tech agricultural race that needs to learn the basics of making farms, planting, growing, storing and surviving.. maybe the locations that work, become original homes for the first tribes.