Well, I think the problem of trying to make DF have effects that fit arbitrarily selected spheres is putting the cart before the horse: We should be agreeing on what sort of effects we want out of spheres, and then making globally dominant spheres that fit those design specs.
After all, there is plenty of reason for there to have been deities of pregnancy and childbirth in the ancient world - pregnancy was a very dangerous thing, and one's entire family line was at stake if wives proved infertile. Those who practiced "magic" in the real ancient world most often sold spells and potions and charms relating to safe childbirth, avoiding robbery, success in business, and other, fairly mundane forms of sorcery...
From the perspective of a Communist Overlord/Deity, where pregnancy is represented as your female dwarves occasionally plopping a new dwarf out of their mystical spore sac, (an act which they can perform while asleep or in the middle of combat,) building a magic system around having a sphere dedicated to making childbirth easier when it is already on total autopilot makes little sense.
This is why I have a problem with "Defining a Marriage Sphere as a union between One Groundhog Man and One Groundhog Woman, to mate for life and produce offspring whose organs can be harvested until such a time as one of the two die, and the other becomes useless for population growth purposes."
If Good and Evil biomes are a placeholder, why can't we consider current spheres a placeholder as well, and just determine what sort of effects we want taking place over our world, and how that would translate into giving us spheres that affect worlds.
Toady actually seems to take this approach, himself, as he seems to have written out how he basically wants spheres to have effects like "making forests that sing" or some such. (Which I disagree with on the basis of nobody's going to notice if the trees are singing or not. The most that would amount to is a little sign that pops up and says, "By the way, these trees are singing... Just trust us on this.")
For example, currently, "evil" means "Oh crap! Skeletal Fire Imps will eat my dwarves!" It also means trolls or grimelings or harpies can show up. We probably want that option to continue. We could have an "undeath" sphere that produces skeletons and zombies. We could have a seperate sphere(s) that produces the trolls and the grimelings and the harpies, and these two spheres may or may not overlap. We could have similar, "fae" spheres that make the satyrs and the unicorns appear.
In this way, spheres continue the role that they are supposed to replace: making creatures appear that otherwise wouldn't, and having a real impact on play options. If a harvest sphere's presence would enable the growth of a wider variety of crops, there's a reason to look for that sphere.
So then, I say again, put the horse before the cart: Let's look for what we want the spheres to do, then define the spheres as that which enables what we want... So, what do we want them to do?