What I'm having trouble with is how exactly to go about the basics of character creation and conflict resolution. How should I differentiate a Soldier from a Wizard, or whatever the player might want to be? How do I balance the choices so that a 2 mana spell to boost your physical stature is equal to one to drain away someone's life force?
Note that I want to base this more on the fluff of the game than the game itself; inspiration should be things like the Urza saga, rather than the cards.
The setting itself would be some plane in the Multiverse sometime after the Phyrexian invasion of Dominaria. Estranged Phyrexian monstrosities roam the planes after the destruction of their home, and the fallout of the Apocalypse is still settling. The plane that the game takes place on was sheltered from most of it by distance, but planes-hopping beasts and strange temporal rifts still plague it as a result of the huge energies released during that event.
Think of the setting as Ice Age, only without the ice, and on a different world.
Here's what I wrote so far.
Each color has 2 allied colors and 2 enemy colors. This does not affect a character's ability to purchase mana of different colors, but creatures embodying enemy colors are rare, and many spells from enemy colors specifically target each other. For example, Black magic is especially good at killing White and Green creatures, and Blue magic is good at disrupting Red.
Black is the color of death, decay, despair, greed, corruption, selfishness, and most every dark urge in the human psyche. Black mana is corrupting, and in its pure form causes rot and decay in everything it touches. Black is the color of undead and of demons, and specializes in spells that drain life, destroy minds, damage souls, create the dead, and various other unpleasant activities. Black mages are often evil, though they are not necessarily so-the corrupting influence of the magic they wield, though, makes it difficult to remain a good person while using primarily Black mana. Supplementing it with other colors, however, makes it significantly easier to keep oneself free of corruption. Black magic is opposed to White and Green magic, and works well with Red and Blue. Black magic is drawn from swamps and other places of decay.
Blue is the color of thought, patience, knowledge, progress, and planning. Blue mana is calming in small doses, but continued exposure can bring about madness and an all consuming desire for knowledge. Blue is the color of traditional magic, and indeed, its core constituency is made up of wizards, rather than any special creatures, like undead or demons for Black. That said, the capricious Fey creatures are either completely or partially Blue, and many summoned or created creatures are as well. Blue magic manipulates the workings of magic, minds, and time itself. Blue magic is the most closely connected with the science of artifice, and many Blue spells manipulate or help create artifacts. Blue magic is opposed to Green and Red magic, and works well with Black and White. Blue magic is drawn from islands and other places that represent a place of calm in the middle of activity.
Green is the color of nature, instinct, and growth. Like Blue magic, Green can be calming, but unlike Blue, continued exposure to it breaks down a creature's higher mental capabilities, eventually reducing them to base instinct. Green is the color of animals, plants, and specializes in the control of natural creatures, inducing growth, and general mastery over nature. Green mages are often referred to as Druids, though not all Green mages are such. Green magic is opposed to Black and Blue magic, and works well with White and Red. Green magic is drawn from forests and other places of nature's supremacy.
Red is the color of passion, anger, rage, fury, fire, and setting things on fire. Red magic is angry, short sighted, and powerful, and its users channel their passion and fury to empower it. Red is the color of the various goblinoid races, of war, and fire, and specializes in breaking things. Red is also the color most likely to summon and control elementals, as creatures of pure fire or lightning are easy to call, and their destructive power can be directed at things simply by facing them in the proper direction. Red magic is opposed to Blue and White magic, and works well with Black and Green. Red magic is drawn from volcanoes, storms, and other places of broiling heat and unrest.
White is the color of peace, justice, wrath, vengeance, honor, and civilization. White represents community and collectives, and is the color of angels. White is not necessarily good; while many good things are represented in White, it is also the color of vengeance, mercilessness, and uncompromising fanaticism. White magic specializes in banishment, pacification, and broad, indiscriminate annihilation. White mages often worship some other being as a focal point for their powers, though rarely does a higher being actually grant any White magic-it is in fact far more common for Black mages to receive power from something greater than they. White magic is opposed to Black and Red, and works well with Blue and Green. White magic is drawn from plains, shrines, and other places of calmness.