I'm just going to bump this to mention constellations:
Groups of stars might be formed together into the shapes of the gods, great heroes, great monsters, etc.
Where a single star might have only a very limited effect, a constellation might have a *lot* of astrological "weight".
Constellations might be formed (hey, I never noticed before, but that group of stars looks like a...) during the life of a Fortress, or even during the life of an adventurer, possibly as a goal (hey, look up there! isn't...is that Steve?!?)--in that a great adventurer might be "set among the stars" by the gods:
Constellations themselves might have souls, sentience, or even physical forms they could occasionally take. Beings could be "born under the sign of", etc.
The waxing/waning of constellations might have an impact on the game. Different species/cultures might have different basic constellations (dwarfs might have the Hammer, Pick, Axe, Anval-while humans might have traditional constellations, like the Big Dipper, the Bear, the Scales, the Hunter, etc. and elves might have the Tree, the Bush, the Shrub, the Pointy-eared Obnoxious Bastard... Goblins might have the Skull and the Phallus (to represent the cycle of death and rebirth), while kobolds could have the Beetle, and other insects, and the Lion (a representation, to them, of death). Water-breathing (amphibious creatures too) might have the Kraken. Even HFS might have their own set of constellations. The Dragon might be universal.
Constellations could be shared among species/cultures, with somewhat different interpretations. Goblins might also have the Kraken, but to them it represents strength in diversity.
The stars would stay the same-interpretations would differ.
Also, astronomical architecture: Constructions might be built in such a way that they bear astronomical import. Massive "calendar" style megoliths could be built, for magical purposes, or just to help your farmers predict the coming season, and when to plant.
Towers etc. could be built specifically as astronomical observatories. Temples might be built "along divine lines", by aligning them with the sky--which could mean building a temple first, in DF mode, and then adventuring in the name of that temple, until Steve McUrist gets set into the sky, thus fulfilling the prophecy.
Also, other planets might very well exist in DF, but I think Mercury through Pluto are already taken.
DF planets could be named after DF gods, though, while the sun (suns?) and moon(s--13!) could actually embody divine power, or the "elder gods" themselves, in physical form. Stars could additionally represent lesser gods, or perhaps "divine powers" that don't concern themselves with your
particular world.
By the way, how many major gods do we actually have in DF so far? I'm familar with Armok, Tsarnok, Tsathoggua...er Toady the Great,
http://en.wikipedia.org/wiki/Tsathoggua and probably ThreeToe, in some form or another, but what other gods have actually been established as canon? As in, "powers that exist in every world, despite the RNG".