How about, a biome where the death sphere is present has a chance for anything dead to come back to life, except not necessarily evil and with a strange hunger for brains. Maybe, if the land is stronfly influenced be some godess whose spheres are Death and Nature, the dead could come back but as neutrals and just hang around in the woods giving happy thoughts to the other dwarves. Except when they come too close and get bitten. Spheres interacting with each other while having an effect over biomes could lead to pretty strange and awesome things, I think.
It could also be hard to implement, but I'll try to imagine something;
To fit in DF it would have to be potentially randomized; so we could associate spheres with objects, materials or statuses. War could be associated with weapons, metal and blood. Death with zombies, skeletons, death. Nature with animals, trees and fertility.
Let's consider Deformity. The definition of the effect of this sphere would be: "Here, people have a chance to be born with strange limbs and body materials"
But when another sphere is strongly present here, they could interact and we would obtain;
Nature; They get random animal or vegetal limbs instead or added to their own, or they are more fertile.
Death; They are born skeletons, or zombies, or with zombie limbs, or with dead limbs (this is atrocious but awesome, also this is actually on-topic)
War; They get weapons instead of limbs, some of their organs are made of blood (causing death)
So basically, every sphere that has an associated object/material which can be placed instead of one of the newborn's , has a chance to create a diformity if Diformity is present.
I got carried away, but the implication here is, undead aren't necessarily just hostile. Maybe they could come back from the dead to help you fight the goblins. Or to help you plant crops. Or kill your priest. Or pull your levers.
... Actually, I just read the whole thread and as it turns out, somebody already had all my ideas.