Another aspect you could speculate on is the possible costs and corruptions for spells associated with each sphere of magic.
Judging from the screenshots and interviews on mythgen we've seen, magic costs seem to depend very much on the world, with the source of magic varying. If the source is a cosmic egg, the costs might be using up tiny egg shell fragments. If magic is found within each creature, the costs might be meditation to reach a certain state of mind, and doing a little dance to release it all. If magic is bestowed by the gods, appeasing them by sacrifices, praise, or sending food up a heaven-pipe might be necessary.
Of course, a spell that involves manipulating a fire still requires a fire, and one that transforms a creature into a tree needs a creature to work. I suppose some settings could necessitate having sphere-specific costs though, such as ones where the magic-granting gods subside on (sphere-specific) sacrifices, or ones where contacting the realm of PREGNANCY requires being close enough yourself somehow to the sphere of PREGNANCY. In general, I don't believe this will be used too much, but including it in this list would do no bad either, and I might also be wrong regarding that.
This also seems to be the same as "Rituals and enhancers" described in the top post, though I don't believe it has been utilized yet.
Corruptions are the same, varying a lot depending on the source of magic. If we want sphere-dependent ones though, there are already the creature effects, or environment/plant/object effects if it corrupts the user's surroundings instead of (only) the user.
I've always gotten an enormous kick out of the idea of magic associated with the Pregnancy sphere, especially with the community's well-known frustration with dwarven children and catsplosions.
Pregnancy
Spell effects:
- Temporarily increase the fertility of a female animal. This would allow them to produce offspring more quickly, but they would still need a male to give birth. This could last for a couple of months or years before wearing off. Does not affect semi-megabeasts, megabeasts, or civilized creatures. Useful for livestock breeding.
- Instantly induce labor in a female animal. Doesn't do anything long-term, but can be done without any male units on the map. Especially useful for animal taming.
- Increase the fertility of a female civilized creature for some period of time. They would still have to still go through the normal channels of marriage or extramarital lovers to give birth, but would do so at a greater rate. Could be useful for creating generational forts when dwarves don't marry at the rates you want them to.
- Instantly induce labor of a female civilized creature. Could be used in combat to severely distract and confuse the enemy, and killing their newborn child could act as a severe morale killer to the enemy if you happen to be a terrible person.
- The most skilled of pregnancy magic wielders can increase fertility of megabeasts and induce labor in them. This could be used to create an extremely powerful dragon army from just one female. Magic users of this caliber could even induce pregnancy in sexless creatures like forgotten beasts.
Weather effects:
- At HIGH levels: Blood rains, alluding to afterbirth and menstration. These have no effect.
- At EXTREME levels: Blood rains, increasing fertility in units that it hits, or can cause instant birth.
Creature effects:
- At LOW levels: Animal and vermin fertility is slightly increased. Both sexes are still required in order to breed.
- At MEDIUM levels: Animal, vermin, and civilized creature fertility is significantly increased. Think, dwarves giving birth every 9 months instead of every year. Civilized units are slightly more likely to become lovers/spouses
- At HIGH levels: Animal, vermin, and civilized creature fertility is hugely increased. Think, dwarves giving birth every six months. These units also experience periodic random pregnancies, which don't require a male. Gelding is the only way to prevent massive livestock explosions. The caverns must be constantly hunted in order to keep creature populations down. Your giant crocodile army will overwhelm invaders from numbers alone. Expelling all couples would be in the player's best interest to keep dwarven birth rates manageable, which will be hard because dwarves will be significantly more likely to become lovers.
- At EXTREME levels: Animal, vermin, civilized creature, and megabeast fertility is massively increased. Dwarves give birth every three months. Dwarves marry or become lovers very quickly. Random male-less births are more common, and even effect megabeasts and sexless creatures. Gelding does nothing. With extreme measures like expelling all females and bringing no livestock, the fort might last long enough to attract some megabeasts, allowing you to permanently increase their number in the world.
Spell effects:
- Ensure the safe pregnancy of a creature for a time period. Does not necessarily involve a safe birth.
Creature effects:
- At MEDIUM levels: Normally egg-laying creatures instead give live births, and thus are pregnant before that.
Plant effects:
- At EXTREME levels: Edible part of a plant causes the eater to become pregnant.
Object effects:
- At LOW levels: A worn object, clothing or trinket slightly lessens the risk of a miscarriage.
- At MEDIUM levels: A worn object, clothing or trinket moderately lessens the risk of a miscarriage.
- At HIGH levels: A worn object, clothing or trinket protects the wearer from having a miscarriage.