I'm looking for a focal concept to focus the development on this new game system. One of the driving concepts in the game is the idea that anyone can learn magic, and that the heroes of the setting are people who have internalised the natural magic of the universe into themselves.
To use the standard tropes of fantasy games...
Fighters become more adept, are able to deal devastating strikes, and might move faster than the eye can see because they have transcended the mundane martial arts of common humanity. Scholars tap into the underlying knowledge inherent in the world, perhaps understanding it as the collective dream, the Akashic record, or spirit guides. Clerics tap into the resonant energies typically associated with their chosen deities. Rogues tap into the mysteries of shadow, manipulating degrees of obscurity by unlocking secrets, or establishing new secrets of their own (invoking invisibility or similar effects in the process). Mages and Sorcerors don't cloak their powers in mundane appearances, they simply manifest their effects blatantly.
Instead of developing specific subsystems for specific types of character, everyone harnesses the same general effects, but those effects manifest in appearances defined by the specific type of character. These general effects include...
Modifiers to action attempts (positive or negative)
Extra actions (or preventing actions)
Rerolled dice (taking the better roll, or the worse roll)
Automatic additional degrees of success (as long as the basic roll is successful)
Back in the late 90s, Dave Chandraratnam and I developed a great magic system structure and it really feels like it could work here. Characters have three magical traits: Connection, Conduit, and Capacity.
Connection indicates the total range of things that a character is able to draw mystical power from. Such things might include places, ritual techniques, specific tools, specific times of day, types of weapon, specific people, herb and crystal types (varying per element), or just about anything else that people might consider significant (and thus attach power to). The specific connection score is equal to the total number of these foci that a character has access to.
Conduit indicates the number of foci that a character can connect to at any given time. A character might have a dozen connections, but might only be able to harness the energies from three of them at a time. Since every connection has a different range of mystical affinity, this means that a character can harness a different assortment of foci to resonate in different patterns to produce different magical effects. However the numbers work out, a character must always have more points in their connection score than their conduit score.
Capacity indicates the maximum amount of magical energy that a character can stored within their metaphysical pattern. This is power that doesn't need to be channeled in from a focus at the time when the magic is unleashed, but it will need to be recharged at a later time. A character's conduit score must always be higher than their capacity.
Latent psychics and the lowest hedge mages may have connection scores but no conduit or capacity. This means they may harness the energies within these foci in the most basic ways, perhaps reading a scroll or following specific prewritten procedures that exploit loopholes in reality known to the occult community. A character with a minimum of one single point of connection can function at this level.
Adepts and sorcerors may have connection and conduit scores, but no capacity. They no longer need to rely on specifically written predetermined effects, and are capable of generating mystical effects on the fly, because they are able to channel the energy as it flows through their metaphysical pattern. Characters of this level do need to have memorised the effects they unleash on the world. They aren't quite knowledgeable or intuitive enough to create custom effects on the fly. A character needs two points of connection, in order to get the single point of conduit necessary to function at this level.
True manipulators of reality have connection, conduit and capacity scores. For them, reality is mutable, energy flows into them, it is manipulated as it fuses with their destiny, then it manifests in spontaneous ways that may be memorised rotes, or new effects specific to the situation. These mystics have started on the path to true enlightenment. A character needs three points of connection, and two points of conduit, to get the single point of capacity necessary to function at this level.
Different schools of magic vary mainly according to the connections they teach. Necromantic schools might draw power from bones, grave dirt, blood, the stroke of midnight, or forbidden texts. Fey sorcery might draw power from dreams, natural glades, herbs, or new artworks. Every school might have half a dozen or more connections, then they might have a dozen specific techniques for manifesting the magical energy unleashed by these foci. Such books would typically have one or two elements of magic in it's pages, its effects will link to a single attribute ("battle"/"intimidation"/"knowledge"/"mysticism"), and a few specific skills to which the magic is related. Magical textbooks might have specific instructions for using two or three foci, and might have a few effect techniques. Often a small magic school might just be a group of practitioners who draw their magical inspiration from one or two texts in their possession. More established magic schools might have generated their own grimoires and texts after generations of trial and error.
Let's consider a book of combat techniques, describing a specific weapon, a specific battlecry, and a meditation technique. Using these, it provides some knacks for dealing extra damage, avoiding armour, intimidation or confusion methods, and more obscure secrets for those who can read between the lines. Such a book might use elemental "fire" magic, the "battle" attribute, and the skills "melee", "intimidation", and "strength" as the basis for its magic. A combat dojo/training-hall might be focused around two or three books like this.