After having wrestled with thise idea for quite a while (currently on my third iteration of magic rules), I concluded that you can't have everything your asking for through current mechanics, for the reasons that others have given.
I've opted to change the power level of magic somewhat (because I run a low-magic campaign, it is very much in keeping with the "feel" I am after).
I use a WP/VP system, and spells are powered with vitality points. Material components can substitute for some, but not all, of these vitality points, and are otherwise unnecessary for spell casting (although some powerful effects would be lethal without them).
Some other variant effects that limit power: first, magic use can be detected at a range of many miles by sensitive persons, meaning spells can reveal the caster. The basic magic feat (arcane ability) allows the character to take other magic feats and also gain ranks in arcane sensitivity (wis), which is a kind of "magical radar" skill that allows the spotting of spells. This rule discourages frivilous casting.
Second, attempting to channel too much vitality in too short a time can cause corruption, which leads to insanity or evil. I set the mark at (primary spellcasting stat) + 1 per four levels. The primary spellcasting stat is a mental stat (int, wis, cha) chosen at character creation. Each VP used in a spell greater than than amount in one round leads to a corruption check (a wis check vs. amount of points over safe threshold plus any corruption points already gained). This rule slows down casting by lower-powered magicians (as they have to spread casting out over several rounds to avoid corruption).
Other feats allow the casting of spells within one magical school (of the standard eight, although some skills require more than one school). I am considering splitting evocation into four feats, one for each major element.
The skills are where the "magic" is. Each skill is a family of uses organized around a theme. In order to work mechanically, I make sure that all powers scale, meaning that more powerful versions of the same effects become available with greater skills. Different effects scale in power differently--some linearly, some arithmetically, and so on. I've found that I need to make up each skill almost completely separately to maintain some balance.
The balances are: limited vitality for the casting of spells (meaning that spell casting makes you much more vulnerable); corruption and detection risks; more flexibility in terms of effects and spontaneity within a smaller range of spells.
The basic spell roll is d20 plus skill vs DC of 10+ # of vitality used, although there are many small modifications. So a first level caster could cast a very powerful spell, then be forced to rest for several hours afterward.
As an example of the system, here is the skill Glamer (visual illusions).
Glamer (int) (requires illusion feat). Glamer allows the caster to create visual images, either static or animated. This skill can create a variety of effects:
Static image: The caster can create a static image of up to small size. The level of detail or intensity of the image depends upon the level used. If this effect is used to create a light source, the radius that is lit is equal to 5' * spell level, with shadows extending half the radius further. Cost: 1 VP * spell level (low progression) + 1 VP per size category greater than small (for larger images). Range: long. Duration: Concentration/short. Save: will disbelieves (+4 to DC for each additional sense that the target successfully interacts with, and –6 to DC for each sense unsuccessfully interacted with).
Animated image: The caster creates a moving image of up to small size. As long as the caster concentrates, the image can be made to move about normally; if desired, the image can be made to do a repeated “loop” when concentration ceases for +4 DC per round of programmed motion. Cost: 2 VP * spell level (low progression) +2 VP per size category greater than small. Range: long. Duration: concentration/short. Save: as static image.
Pattern: The caster creates intense swirls of colors in bewildering patterns, causing disorientation or blindness in targets that are susceptible. All targets within the area of effect except those blind or not susceptible to mental spells must make a will save or be dazed for a number of rounds equal to the margin of failure; all who fail by more than six are blind for a number of rounds equal to margin of failure –6, then dazed. Cost: 1 VP per level (high progression). Range: 10' cone, medium progression VP cost or DC increase to increase range. Duration/Save: see text.
Illusions are easy to create and relatively low power, but their ability to change reality is somewhat limited. If you attempt to use them to directly affect another being (through patterns), the vitality costs are much higher.
Low progression levels match level: 1 VP for level 1, 2 VP for level 2, 3 VP for level 3, and so on. High progression increases the cost by the level per level: level one is 1 VP, level 2 is 3 VP, level 3 is 6 VP, and so on. The level determines the DC of the saving throw.
Sorry to go on so long. I've got parts of the system worked out (in Microsoft Word, I'm afraid), but am still working on the various spell skills, which would be the heart of the system. It plays fairly well in testing so far.
I've opted to change the power level of magic somewhat (because I run a low-magic campaign, it is very much in keeping with the "feel" I am after).
I use a WP/VP system, and spells are powered with vitality points. Material components can substitute for some, but not all, of these vitality points, and are otherwise unnecessary for spell casting (although some powerful effects would be lethal without them).
Some other variant effects that limit power: first, magic use can be detected at a range of many miles by sensitive persons, meaning spells can reveal the caster. The basic magic feat (arcane ability) allows the character to take other magic feats and also gain ranks in arcane sensitivity (wis), which is a kind of "magical radar" skill that allows the spotting of spells. This rule discourages frivilous casting.
Second, attempting to channel too much vitality in too short a time can cause corruption, which leads to insanity or evil. I set the mark at (primary spellcasting stat) + 1 per four levels. The primary spellcasting stat is a mental stat (int, wis, cha) chosen at character creation. Each VP used in a spell greater than than amount in one round leads to a corruption check (a wis check vs. amount of points over safe threshold plus any corruption points already gained). This rule slows down casting by lower-powered magicians (as they have to spread casting out over several rounds to avoid corruption).
Other feats allow the casting of spells within one magical school (of the standard eight, although some skills require more than one school). I am considering splitting evocation into four feats, one for each major element.
The skills are where the "magic" is. Each skill is a family of uses organized around a theme. In order to work mechanically, I make sure that all powers scale, meaning that more powerful versions of the same effects become available with greater skills. Different effects scale in power differently--some linearly, some arithmetically, and so on. I've found that I need to make up each skill almost completely separately to maintain some balance.
The balances are: limited vitality for the casting of spells (meaning that spell casting makes you much more vulnerable); corruption and detection risks; more flexibility in terms of effects and spontaneity within a smaller range of spells.
The basic spell roll is d20 plus skill vs DC of 10+ # of vitality used, although there are many small modifications. So a first level caster could cast a very powerful spell, then be forced to rest for several hours afterward.
As an example of the system, here is the skill Glamer (visual illusions).
Glamer (int) (requires illusion feat). Glamer allows the caster to create visual images, either static or animated. This skill can create a variety of effects:
Static image: The caster can create a static image of up to small size. The level of detail or intensity of the image depends upon the level used. If this effect is used to create a light source, the radius that is lit is equal to 5' * spell level, with shadows extending half the radius further. Cost: 1 VP * spell level (low progression) + 1 VP per size category greater than small (for larger images). Range: long. Duration: Concentration/short. Save: will disbelieves (+4 to DC for each additional sense that the target successfully interacts with, and –6 to DC for each sense unsuccessfully interacted with).
Animated image: The caster creates a moving image of up to small size. As long as the caster concentrates, the image can be made to move about normally; if desired, the image can be made to do a repeated “loop” when concentration ceases for +4 DC per round of programmed motion. Cost: 2 VP * spell level (low progression) +2 VP per size category greater than small. Range: long. Duration: concentration/short. Save: as static image.
Pattern: The caster creates intense swirls of colors in bewildering patterns, causing disorientation or blindness in targets that are susceptible. All targets within the area of effect except those blind or not susceptible to mental spells must make a will save or be dazed for a number of rounds equal to the margin of failure; all who fail by more than six are blind for a number of rounds equal to margin of failure –6, then dazed. Cost: 1 VP per level (high progression). Range: 10' cone, medium progression VP cost or DC increase to increase range. Duration/Save: see text.
Illusions are easy to create and relatively low power, but their ability to change reality is somewhat limited. If you attempt to use them to directly affect another being (through patterns), the vitality costs are much higher.
Low progression levels match level: 1 VP for level 1, 2 VP for level 2, 3 VP for level 3, and so on. High progression increases the cost by the level per level: level one is 1 VP, level 2 is 3 VP, level 3 is 6 VP, and so on. The level determines the DC of the saving throw.
Sorry to go on so long. I've got parts of the system worked out (in Microsoft Word, I'm afraid), but am still working on the various spell skills, which would be the heart of the system. It plays fairly well in testing so far.
