I kinda like this system. I dub yee system, Fullmetal Summoner.
This system relates itself to the idea in Shinto that there is a spirit in everything. The questions you need to answer are thus:
1. How often can a Fullmetal Summoner summon a spirit?
2. When you have 2 summoners in the same place at the same time summoning the same spirit, who does the spirit listen to?
3. Have Summoners in your world made a sort of Mystical currency for summoning or do they play it by ear all the time?
4. Going with the Shinto analogy, are there a finite or infinite number of spirits in a given area?..
Let me rephrase, are there restrictions for summoning spirits in an area?
For example, could you summon a spirit of fire in the middle of an arctic tundra without even two sticks to rub together?
Do the spirits need a link to the real world to be summoned or can you just pull out the Spirit Yellow Pages and summon whatever wherever and whenever you need it?
That's all I got for now. I hope Fullmetal Summoner pans out.
So this train wreck of mine has a name! Thank you. I'm honored.
To set about answering your questions:
A catch all answer would be "it depends." lol. I'm trying to treat magic as an art, rather than a science.
1) Summoning is difficult. I don't want to set a maxim, but it's not something taken lightly. I think the more important question would be, "how often are you willing to risk losing life, limb, and possibly your soul?" A person can try to summon a spirit as often as they want, but the process is physically and mentally taxing. Liken it to taking the MCAT, knowing that if you don't score high enough you could die (or worse.) How often can a person put themselves through that? If you spend hours on end mentally focusing yourself to achieve this sort of thing, it's going to run out your battery. Some people are stronger than others, and some sorcerers have trained themselves to become more efficient.
2) The spirit will usually listen to whoever's got a stronger force of will. Of course, a blood sacrifice can be made to help one's cause. (My MC's demon familiar/sidekick/lover interest explains that when anything dies, it screams, on a spiritual level. This "scream" is used to fuel a summoning and entice something from the other side.)
I think it's unlikely that two people would try to summon the same spirit. There're countless demons and spirits in the netherworld, so I don't think two people would often try to summon the same one. Even if they did, physical location would be irrelevant, since they're trying to call it into the world.
Your comparison to Shinto is a good one, but it doesn't take into account the idea that summoning involves breaking ::through:: the veil that hangs between earth and the netherworld. However, small spiritual essences can come across into our world, and do frequently. Think of minnows slipping in and out of a fishing net. These account for ever-present spirit essences alive in the world. To achieve magical effect, you're usually going to need to invite something stronger into our world.
As I write this, I've been considering and now I think that certain stronger entities should exist in the physical world. Nature entities especially, tied to forests, mountains, ocean, etc.
3) Sorcerers can use rituals, and chants and magic words, but there're no hard-set rules. It's like sex. What works for some people doesn’t always work for others. Blood sacrifices are often helpful. A group of people can pool their will to work towards a common goal.
4) There would definitely be a finite number of spirits in any given area, and more often than not, they're not strong enough to really be of any use to a sorcerer. Thus the sorcerer would need to summon it into the physical world from the nether realm.
In my universe, a netherworld exists. My main character's demon familiar calls it "The Dark Place." In essence, it's hell. But there is no Satan, no angel rebellion. It's a dark, frigid place that serves as a dark mirror to our own world. Spirits, demons, and shades dwell down there.
They can come across of their own accord sometimes, but as a rule, the stronger something is, the more difficulty it has remaining in our world. The MC's demon familiar likens it to walking across thin ice. Once something comes across it inevitably "falls" back down to its home. This can be mitigated by forming a pact with a sorcerer. The covenant serves as a lifeline for the entity. Demons are usually the most willing candidates because they have human drives and desires, whereas some other entities are just so alien, it's hard to reach any kind of understanding with them. Once this link is formed, the demon or entity can remain in the world until the stipulations of the pact are fulfilled. Some sorcerers form lifelong pacts.
To answer your question, you could certainly summon some kind of entity to make a fire in the frozen tundra, but again, you must ask yourself "is this worth it?" Can I control the thing that I bring into this world, and for how long? Will I freeze to death if I don't? My characters will have to universally recognize that it's always easier, safer, and more efficient to use mundane methods to achieve their goals.
In regard to the "spiritual yellow pages," I see summoning as a bit like fishing or hunting. You may have a very specific idea of what you're trying to angle, or shoot, but there's always a risk that something else might show up. As a real-life example, my uncle once got chased up a tree by a bear when he was deer hunting.
I'm still working on the idea, and this "system" (I use the word loosely) is purposely a bit nebulous. I don't want to set too many limitations, or create something that's methodical. I want to keep a mystique about it, and I want magic to be strange and a bit frightening, even to those well-versed in its use.
At the same time I don't want the MC to pull some ridiculous magic spell out of his @ss at the last second and save the day. I recognize the danger in that, and have decided that dramatic conflict in my stories should be resolved through human endeavor rather than some all-powerful magic spell.
Thanks to everybody that's offered advice so far!