As far as I can tell, magic is always rare and mysterious, and terrific, but beyond these commons elements, I see 3 kinds of magic.
Sorcery is ritual magic, it has a high cost to learn and to use, but is very powerful and the effect are beyond coincidence. I mean that can't be explained without magic. This are the shape shifting abilities of faceless killers, Melisandre shadows, Miri Maz Dur "resurrection", Thoros flaming swords, and so on...
Sorcery is, I think, the easiest to design, as it is spell bound. Designing magic is only designing a mere list of spells, with requirement, costs and effects. Of courses there's a need to restrict a player from knowing all the spells, and it should be interesting for the GM to have guidelines to create new spells and effects. We've seen that in other RPG before, and it's not that different than any other sword and sorcery universe.
Still there's and obvious need to remove any power/magic point system, or any spell slots system and to replace this by a darker cost, so cost for each use of magic that will make the player wonder if he will use the spell or not. Because spirits and powers won't work for free.
Gifts on the other side, are abilities that goes beyond the usual thinks, but that are merely an incredible mass of coincidences. I mean no visual effects, no proof, but the power is real. Bran his a Warg, but a septic master could state his only ability is to dream he's a wolf, and that, by chance only, the wolf do as Bran dreamt... And we could say the same with the ability to foretell the future, which is usually cryptic and subject to more than one interpretation... I think reading through the books would bring us other abilities belong greenseer, warging, and prophecies. Just like this young boy Baelor names as a High Septon and who was known to heal wounds by laying hand.
For me, Gift are innate, You have them or you don't, but you cannot become a Warg if you're not born as one. Training doesn't help much, and it's I think the main difference with sorcery. Another difference is that you don't control the power as much as it controls you. Sometimes, you don't choose to use it, and it becomes active by itself. The "spellcasting" is intuitive.
So I think this gift should be only attributed at creation, or during special character transformations (like loosing legs) with GM approval. There should be a skill to gain control on the power, but I don't think there should be absolute control. The GM can always activate the power to fit the story, and the player shall not be certain that he has control over it...
And this is especially true for fortune telling and foreseeing: Usually powers like dreams are used by the GM to give the player hints on things, but not by the player to investigate or understand. So it's usually a power to help the GM, and not the player, and hence asking the player to spend times and experience point developing it seems quite hypocrite.
Miracles at last, are unusual events, induced by a specific character, at a specific time, and with absolutely no control of anybody on it. That's an incarnation of Fate itself. You can't make it on demand, and you can't make twice the same. That's obviously the dragons awakening for Daenerys, but there's other illustrations of theses. We have Baelor the Blessed walking unharmed though the snakepit. We have Catelyn praying for Bran to stay in Winterfell and bran staying there after falling from the tower.
Miracle are created and controlled by the GM, and are created in the way that best fit the story. But Miracles come from characters faith. Dany knows for sure that dragons are alive; Baelor and Cat share the belief that the sevens watch over their children...
Likewise I think the player should trust the GM.
I think this kind of effects can be design in a role-playing system. Here is an example: When the character have a deep faith in something, the player gives some experience point (or whatever else valuable, like burning is old friends) when the players need a miracle, the GM looks at the point he have, at the story, and at his inspiration. (Is there a tenth muse for rpg?) Then he decides whether he has enough points to perform a Miracle for the player or whether he prefers not to do so. If he does, he will burn as much "faith points" he would like to build a miracle as spectacular as he think he should. Of course, there should be a few guidelines and rules to help the GM, but in the end, it will rely on trust between the player and the game master, and of the mood of the game master. I think that should be perfect to simulate the trust a character might put in the power-that-be, and the mood-shift of those powers...
And as a mater of fact, this mechanism seems to be a perfect way to handle curses.
I hope this will help.
And by the way, what are the many topics you're discussing? I'm sure there's a lot of people here who would be glad to brainstorm on them. (Even if like all the storms it would produce a lot noise and confusion...)