Innistrad, being part of the Magic: the Gathering multiverse, is inundated with magic. In MtG, there is no difference between arcane and divine magics like we are accustomed to in D&D. Instead, magic is flavored by its mana source, of which there are five options: white, blue, black, red and green, the mana of which is garnered from lands like plains, islands, swamps, mountains and forests, respectively. Priests of Avacyn, on Innistrad, would naturally believe their healing and protective magic is granted to them by the angel Avacyn herself. Rather, they just simply do not fully understand the source of the magic.
This line of thought naturally will cause a change in the way I described magic in my last post, but this is more a thought experiment to see what I can conjure up for this game.
So, by eliminating the clerical options, and instead expanding with the five different colors of Magic, we are offering the players significantly more choices with their character progression. We'll retain the "have to have used it to acquire it when you level" rule. What this means is that any magical item found in the game will have to have a color identity of some sort.
I would also propose that when a wizard gains a level, they have to physically make a connection with the land that is the source of their particular color of magic. They cannot cast their spell on their own until they do this! Mana flows from the land, and they must be bonded with it to use it.
This is the kind of thing that will require a ritual of some sort, of course, each of which would be different depending on the type of mana.
So, the basics of each of the five types of magic.
White magic comes from the plains, and is often protective or healing in nature. White magic all about justice and law. D&D's clerics would use white magic.
Blue magic comes from islands and the sea. It often works on the mind, with illusions and deception. D&D's illusionists would use blue magic.
Black magic, from swamplands, is as bad as it sounds. It is the magic of death, decay, selfishness and raw power. Black magic is the province of necromancers.
Red magic, of the mountains and highlands, is chaotic and full of unfettered emotions. It is often manifest in flames.
Green magic, from the forests and woods, is the magic of nature. The druids of D&D would use green magic.
Choosing a color of magic at one level does not preclude a mage from choosing a different color at another level (as long as they have experience using magic of the particular color). It does, however, mean they will have to forge another bond with a different type of land. Gaining a level in a second color will not be difficult. However, the third, fourth, and fifth will be. There is a chance the land will reject a mage, and the ritual fail.