since there is caverns and hell, and sometimes players build mega projects so far up that they cannot build any farther up into the sky. I was thinking of this: making a "cloud" layer above the sky that is mine-able and about as easy to mine as soil. There will be a couple of cloud layers with the third most significant one being heaven.
Various creatures spawn in the cloud layers just like in the caverns, but most of them being good, like winged unicorns, giant fluffy wamblers, etc... with trees and plants being good-aligned trees like feather tree and sunberry in addition to some new ones exclusive to heaven and the cloud layers. Plants from the skies wont grow on the ground, but will grow on muddied floor tiles that are a certain number of Z levels above the ground.
Once heaven itself is breached (which will be for no reason at all, except for fun, as hell is breached from greed for adamantium) your fort will be besieged occasionally from the sky by flying war-angels, heavily armored crusaders of the light, and lightly armored sky-elves because they consider all races of the earth (except for maybe elves) to be impure, and not completely "good". The sieges will have the message "The Enemy is Upon us: A Radiant Force of Light has arrived".
The siege may arrive in the cloud layers, the layer of heaven, or maybe descend to earth first and then advance like a normal siege. At it's worst (funnest!) the siege will be composed entirely of fliers that will fly down to the earth levels but not "walk" to your fortress, and instead fly.
Enemies and creatures that naturally occur in the clouds might also possess the ability to transverse the solid clouds freely as if they were air, making defending and fighting in the cloud layers more dangerous (fun) unless you construct everything there out of materials of the earth (and not simply mining and walling off entrances with bridges).