I still like the concept of drawing mana out of the environment somehow (rather than drawing it out of thin air) which I added in these examples. It makes the cost/benefit a lot easier to balance, gameplaywise.
There could be a map of ambient magical potential for each kind/sphere/source of magic. For example, fire magic is easier/stronger near fire, so terrain features related to fire give a bonus to water magic nearby (obvious ones are volcanoes, geisers, wood, but also materials, shapes or symbols associated with fire in that world (eg. sulphur, obsidian; triangles; imps).
The result would be that fire wizards seek out places near volcanoes, wood, preferably with sulphur and obsidian in the underground and build a triangular dwelling there, surrounded with statues of imps.
The same for water wizards in swamp deltas, with drop-shaped dwellings and quiksilver bassins, or air wizards that would build towers on mountain tops and cliff edges, death wizards on elephant graveyards in deserts etc.
A random example: the power source is jewelry, the associated terrain features are oyster banks; the associated materials are precious metals and gems; the associated shapes and symbols are squares, lemmings and the color purple. Result: the magic-user sets up his camp at a place where the ocean has eroded a cliff: on the cliff are lemmings, in the cliff face silver veins are visible, and at the bottom are oyster banks. He erects a square dwelling, which he dyes purple to enhance his magic even more.
The spheres of magic would need to be predefined with the associated terrain features. All other associations can be randomized at worldgen, though it should always be possible to restrict them. This way the AI can use the symbolism as guide, and it will give a distinctive flavour to the various magic users a player might encounter - or play.