Not a bad idea, seasons are already linked to certain concepts. Besides things like hot, cold, rain, and snow, there's the spring/summer/fall/winter metaphor for birth/life/decay/death.
I had an idea a while back for a sort of "magical day" system, which could either matter in adventure mode, or be adapted for a year-long cycle of the same sort. Basically, think of "magiceyness" building up (slowly) under sunlight, peaking at dusk, and decaying until dawn. You could have nocturnal and diurnal creatures, but also creatures that prefer a time of magic or lack of magic. Their days would be measured in noon-to-midnight instead of dawn-to-dusk.
A creature that prefers magic, but darkness, would be strongest dusk-to-midnight. A magic-hunter would hunt midnight-to-noon, when his prey is the weakest, but a solar-powered detector would only work half that time.
Bonus time! A werewolf (changing involuntarily based on the moon, day or night) that feeds on magic and prefers to operate at night would have an even more complicated system of strengths and weaknesses! And even more so if it's human side operates differently than the wolf side! Being affected by tides (and therefore the sphere of water?) is an alternate way of being affected by the moon, possibly even if underground.
EDIT: Second fairly big idea. So, HFS is the endgame of your fortress, right? But you're supposed to keep the larger world going after that... what happened to the infinite... things? HFS could be a sphere like everything else, and just be VASTLY INCREASED by accessing those portals... but have some way or another of being resealed, so they entire world doesn't end the first time someone digs a bit too deep.
Bogeymen and other such things could be an effect of a moderate amount of HFS sphere, but not as much as actually opening a portal? Maybe it'd make magic in general easier (or certain kinds at least), encouraging players to take risks, maybe meddle in a second area they "really shouldn't"?
EDIT2: (pulled from another thread)
DF isn't the sort of game where you can paint a black and white morality over it. (Where "good" means a normal place, but with unicorns to impale your hunters instead of just hippos.)
This might have more place in the game if we had sphere-based surroundings rather than the placeholder "good" "evil" and "savage"
Just as a suggestion, how about "good" becomes "high magic" or such, evil is either corruption or necromancy, and savage is primal? A bit semantics on the last part, but "savage" sort of implies "stupid" to me, where "primal" is just "very strong and super-alive".