Apologies to the others for not responding very promptly to your own comments, I became somewhat distracted.
When it comes to fields, I would think there are at least two kinds of fields:
- Powered, i.e. some kind of power source maintains the field, and that source will deplete and disappear over time ("magic crystals", both stationary and portable. They may or may not be possible to recharge (blood sacrifice is a classic method, but the method ought to be in line with the fields itself).
- "Permanent", akin to black smokers in that they provide a more or less variable constant power output. "Permanent" sources may be destroyed/shut down, both via natural fluctuations and through intervention, and their sources might not be truly unlimited, but they are over the time scale of fortresses.
I don't see the reason to conceptually differentiate these. (Amusingly, the "black smoker" volcanic vents only last for about ten years or so at a time, the crabs and worms have to move from generation to generation.)
Let's say that every...
thing that produces or consumes xenoenergeia does so every 100th tick outside of magical actions like spellcasting. What's the real difference between a creature with a +1 and an artifact with a +1? Sure, maybe some of those "run out of energy", but creatures also just plain die, and artifacts might be destroyed. Aren't those all conceptually equivalent?
As far as I'm concerned, DF already have fields in the form of the various cavern biomes, as well as the good/evil ones, plus well known everyday ones in the form of temperature, precipitations, etc. (Additional) magic fields would, in themselves, basically result in new biomes (with different combinations of new fields and levels of existing ones leading to various new biomes). This, in itself is interesting, but it becomes even more so if you (and other agents) have means and motivations for changing/manipulating fields.
Keep in mind that part of why this idea exists is in response to Toady's desire to phase out Good/Evil/Chaos biomes in favor of sphere ones. Presumably, the myth generator will do this, replacing these biomes with areas around cosmic egg shells or something.
One could e.g. imagine sapient magma creatures who wanted to expand their domain upwards (by melting non magma safe rock, layer by layer). Firstly, they'd want to prevent the main source from being weakened (resulting in lower magma levels) or shut down (resulting in obsizianization or draining of the magma sea), but it ought to be incredibly difficult, or even impossible, to achieve that. To expand the domain, they'd either have to increase the output of power from the source (removing blockage to restore or achieve maximum output, probably), or, usually, through artificial means, such as rituals (short term effect, so they'd have to be repeated again and again), opening conduits to the plane of magma (that would require effort to keep open), deployment of magma field "crystals" (which would have to be "manufactured" at a considerable cost (in various currencies appropriate to the fields) and would deplete over time), etc. When the efforts to depart from the "normal" state cease, the field would ease back.
Underlying this, there may be natural processes that shift the fields around (think tectonic plate movement or erosion), that may or may not operate on DF fortress level time scales. You could also have manipulative gods in the mix...
If you used this kind of logic, it would make sense to be able to affect the local temperature, precipitation, or other "non magical" fields through magical means.
That depends in some part on whether they are considered "magical" or not.
Magma might be just magma, and magic related to heat might not connect with actual surface temperatures. It may in fact depend a lot on what "magic setting" you have. Maybe all "physics" becomes related to a magic sphere at high levels, and spheres definitely don't have any practical effect at the low end. In the middle, however, there may be some open question about whether mundane fire has anything to do with the FIRE sphere, or a magma forge has any relation to a MAGMA sphere.
Sacrifice to the magma gods might do nothing but power magma magic, but have no practical effect on actual magma. In fact, magma magic might just be restrained to within touching distance of magma, if it's "field" is instead bounded by its physical representation, rather than its physical representation being bounded by its magical field.
However, any harvestable resource supported by particular field strengths would probably have to be either "organic", i.e. grow, or be some kind of condensation/precipitation/... on top of the surface of the area (which may be cavern floors or walls, not limited to the above ground surface), rather than messing with the underlying geology (increasing the magic X field inducing magic X gems to form in rock, and have those gems disappear when the field is reduced gets messy).
Apart from magic field strength manipulation, you might have "conventional" magic that requires the presence of the fields to work, but then you move over towards the "cause-effect" type of magic.
It seems pretty clear now that sort of "cause-effect magic" is what we're getting, and xenosynthesis allows for a more rational way of handling it without using MP or just infinite magic powers. Still, I tried to present it in a way that would appease the more dead-set "no magic dwarves" crowd, (I believe the word "midichlorians" was used...) and it can work for that sort of game, as well.
Considering as Toady's current system involves "sacrifice two plants and become slightly faded" as magic costs, and he talked in the GDC speech about how magic powers might only work in close proximity to certain magic artifacts like egg shells that create "magic biomes", this idea of xenosynthesis seems rather compatible with what Toady is doing. The main difference being the system where changes can occur, whether through player action or mere simulation causing death of biological actors upon this magic ecosystem.
To go all the way back to magic plants, however, the main idea behind xenosynthesis is to make plants and animals in some way a converter between magic energies and standard physics' energies, the way that real plants convert sunlight that would otherwise turn into waste heat energy into complex chemical energy that animals can consume and use, while burning calories and decomposers gradually break everything back down to base chemicals and heat.
In a sense, it's just a way to convert science we all can understand into the magic system, and vice versa, which I believe would be critical for DF thematically. DF is the game it is because of emergent gameplay. Even relatively simple systems, if enough of them interact together, produce extremely complex and difficult-to-predict emergent outcomes, and this is the core of DF's appeal. A magic system that sits off in an isolated corner of the game with no impact upon or relation to the rest of the game's systems drains out this key strength of DF. Making magic fields interactive with herds of migratory animals and your farms means that there's an on-the-map-interactable component of the magic system even when you're not casting spells from adventurer mode. Likewise, having your character's spellcasting in Adventure Mode drain some aspect of the surrounding magic field that is maintained by a finite number of local critters you might also be killing is a direct consequential impact of your choices that goes beyond mere "MP bars".
Besides, I've always liked base-building in games like Terraria, so making a wizard adventurer have to set up a little farming for magic mushrooms/seeded crystal growths/flocks of magic-eating, metallic-magical-egg-laying geese in the basement to serve as "magic ammo" for spells is great fun for me.