I'm seeing a lot of great stuff here and thought I'd make a few comments before unveiling a couple of ideas I have.
Im no writer, but I was thinking along the lines of calorie based magic. IE: how much you weigh determines the degree of difficulty of the spell.
For example, a 200 pound man could not cast a spell that used up his 200 pounds or he would die.
The fatter you are, the bigger the spell you can cast, but you lose the weight and have to gain it back.
I love it because it is such a tangible mundane Source of fuel, and brings up issues involving peoples Prejudices. A well written book with this mechanic would have allot possibility!!
People would study the process just to burn off calories to maintain there figure and eat whatever they want. Others would get huge to store up reserve energy. Also hitting them with some permanent or temporary issues like hanging extra skin when the 800 pound man creates a new continent and becomes a 120 pound pink koloss . It could easily be foreseeable that being overweight would become illegal (or cost extra for a air plain ticket ) do to fear of what an obese person might be capable of unleashing on the world or more importantly the controlling powers (government). Those who control the world would keep the process secret while vilifying unhealthy people. Possible propaganda “Respect your body to show your respect to the creator” and “your body is your temple by disrespecting your health you disrespect the lord ruler" .
As you can see from this individuals (my) opinion who looks at everything threw his game master/dungeons master / keeper's eyes it would work fantastic. I would run a campaign using this mechanic the rest of you should wright the book .
I like this idea a lot, but one thing I might bring up is that when the the bigger you are the more power you have and people are attracted to/intimidated by power, this will skew people's perceptions. Weight might be considered more attractive, a big man would walk down the street without fear of being mugged, etc. One thing I might actually be better at is considering possible psychological implications of different magic rather than actual magic.
Forgive me if this sounds a little childish, but I was much younger when I came up with this one, and I really liked playing LOOM (old LucasArts game).
The magic system is based on an alphabet (probably not English, but I'll use it to illustrate). Your tool, be it staff, wand, sword, house, whatever, would be divided up into the letters A-Z. At first, you would only have access to a few letters, say A-E, but over time you would learn new letters and expand your 'vocabulary'. Spells would consist of a variable length of letters depending on complexity, number of targets, power of spells. More difficult and dangerous spells would contain higher letters, with possible earth-changing spells being up in the X-Z range. So far, pretty simple.
What makes it more interesting to me is that the letters are transferable, to a certain extent. For instance, if you had a staff with A-E, and you found another implement with F-I on it, you could graft them onto your existing staff and get instant access to those letters (although you still may not know any spellwords for those letters). Now, a given person may have really powerful letters, and weaker ones, based on how often they were used for spells (it could even be that certain letters were reserved for certain categories of spells—healing uses primarily vowels, attack magic uses hard consonants, defense uses word pairs like th and sm, etc.). So, you might find an old wizard's staff and discover that his K, M, and P were really strong, but most of his other letters were mediocre at best.
Building on that concept, there would come a point where a letter would be reclassified to artifact status. This being, it's really freakin' powerful. Any spells created with that implement using an artifact letter would have vastly superior capabilities. Trying to build an implement with as many artifact letters as possible has resulted in numerous wizard wars over time, with many murders to steal others' letters. There are lots of potential story arcs that I can think of with this, with the simplest being that a dire need has arisen for a really powerful complex spell (say, using Z), and the hero needs to find an artifact Z to complete the spell properly, even though his staff isn't even to M yet. He has to not only complete his staff to Y (in order to attach the artifact Z), AND find a letter Z powerful enough to use (which would be found only on ancient wizards or on implements that have been passed down through many generations), but the rest of the letters need to be powerful enough to complete the spellword, which he needs to learn as well.
Anyone want to drag this one into the dirt? I like criticism.
This is good, but one recommendation I'd make is changing it from being the roman alphabet (the ABCs) to a set of runes, similar to Brandon's Steel Alphabet, but with more in-world applications. This also removes the limit of how many runes you can have and how powerful the "highest" level of magic actually is. You could have undiscovered runes all over the scale. You could have your story where the "Z" is thought to be the highest level of magic and needed to save the world, and it does, but there's more. The characters could discover that the rune they had before was more of a "T" with even more powerful things to be discovered.
@Kaz: There's a good magic system that blends together magic and science in creative ways, and then there are systems so mired in science that the magic kind of disappears. I'm not saying you fall into one or the other, I'm saying be careful not to make it too complicated.
All right, I've put out my two cents on other people's systems. Not the others aren't good, I just didn't have any particular thoughts on those. Now here are a couple I came up with while reading through here! Now, these particular ideas focus less on what the magic does as much as how properly using it works.
I've seen a couple of ideas on music based magic, so I extrapolated the concept a little and came up with what I think may be good set up for costs on such a system:
Music Magic
Performing music while accessing your magic allows you to cast spells, but the less precise you are while cast a spell, the worse it goes, especially during important parts of the tune. Different types of music do different things. The emotional range, length, difficulty and number of parts/instruments affect the level of power and complexity, but the more important a note is, the worse things can go if you screw it up. If you're trying to play a major scale to start a fire and one of your note goes sharp the fire could go out of its specified bounds, potentially hurting someone. Your epic, orchestra piece meant to restore the planet's life-force could be going great, but at the climax of the song, your soprano soloist goes flat on the big note, causing a world shattering earthquake. The bigger the risk, the bigger the return. Precision is key.
Now the idea of thought based magic is an interesting one. It can be used in a lot of ways. The biggest issue is how much control someone has over the magic. I like Eragon, I'm not afraid to say, and an interesting bit of backstory they gave on the magic system is that in ancient times the only thing that limited a person's magic was their own mind and the other rules already defined in the series, which was the reason they put limits on it, in this case a language to cast spells through. But what if that magic was completely without limits, but had extremely high costs?
Thought Magic
You can make anything you think a reality, if you're willing to pay the cost (temporary brain death? loss of self control?) and have the concentration not to let your mind wander while casting. A single stray thought can shatter everything you were trying to accomplish. Most mages spend years, decades even, mastering their minds to use this magic. Even with this heavy training, the costs can be high. Discipline is key.
I've been tinkering around with the idea of instant magic. I haven't worked out the limitations and specifics, but I think I like the concept of being able to do almost anything... and it barely lasts a second. You can stop a blade aimed at you, but only if you create your shield at the right moment, at the right place.
Maybe this could be combined with my above idea, one of the best ways to keep concentration is to only use magic in extemely short bursts. The length may not be an enforced rule, but simply a useful and common tactic. Part of what makes a master of the magic could be that their discipline allows them to use extended amounts of magic without flinching.
I've always wanted to see a magic system that isn't catastrophically powerful like the One Power or Allomancy. A magic system that relies mainly on the resourcefulness of the user.
You mean something like this? Note: Yes, I read Bartimaeus, it was really good. This was a partial inspiration.
Demon Magic
A mage casts spells by summoning a demon to do what he designed the spell to make it do. The focus is not of what it does, but the cost. In exchange for the task, the demon is given freedoms depending on the power, difficulty, type and scale of the spell. This makes every spell a risk. Is it worth it to cast a spell to lift boxes when the demon who does the heavy lifting will go on to smash up a good chunk of your warehouse? There is also the possibility that the spell is simply not enough to contain certain demons who can only come through the most demanding of spells. Summon a mind controlling demon to turn the enemy army on itself? You might end up with it ruling your country since a demon with power on such a scale is both clever and strong enough to escape the bonds of nothing short of the most skilled sorcerer. This makes the user look for alternative solutions to their problems that either don't require magic or allow them to summon low power demons. Instead of an epic level mind control demon you could summon a demon who simply looks like he could wipe the army in one go and make them break ranks through sheer intimidation. Resourcefulness is key.
The last system is the one I'm most interested in, but I think the others have a lot of potential. My big issue is classifying exactly what they do.