I'd bet on Pendragon doing a better job, HH. It fits the general time period better, it's set up for a dynastic game, and it includes an opportunity for death from sickness for the scions of a noble house-which all fits GoT to the letter.
Ok, can wait.
That said, how would I do it with d20-style system? Well, reskinning what isn't banned, and banning what's not reskinned is the name of the game. First off, banning.
We ban races, except humans. 'nuff said.
We ban all the PHB1 classes, except the bard. The bard gets, well, Bardic Knowledge, but he also starts getting spells 3 levels later than normal. No cantrips before that, either. Instead, they get 2 Skill points per level (8 at first) more. Because bards casting spells comes from Celtic tradition, and Celtic tradition isn't a major influence on ASoIaF.
All broad-magic classes are banned (especially wizards). There's a reason for that, and it's a simple one: specialisation to a theme (the theme usually being linked to how the powers are acquired and manifested) is common in ASoIaF. In fact, it's a defining theme of all similar settings. (Want broad, reliable magic? You're not playing anything remotely ASoIaF-like). Want your magic to be able to handle everything? Play another genre.
Oh, and spells above 3rd level? They don't exist as spells. Instead, you can potentially achieve the same effects with a ritual. Except it requires tough to get ingredients. (If you get two rituals in one session, you're making it too easy to obtain them). Blood of kings is a popular one for certain people, and it entails permanent loss of Hit Dice. This can result in death.
And of course, the spells above 6th level don't exist, period. Wishes aren't spells, they're what beggars get when there's a siege.
So, what do we get instead?
Barbarian remains as a strictly NPC-only class. Because it fits Tyrion's men, and for that matter, it fits The Riding Mountain.
Everyone gets one level of Sneak Attack, and one level only, with their first level. Don't get flanked.
Instead of fighters and barbarians (all the PCs should have some combat ability), we introduce Tome of Battle. No, these aren't mystical classes, they're normal swordsmen who have actually learned to use more than "I strike, I strike again" maneuvers...which is to say, normal swordsmen, period.
Crusaders and Warblades get to be knights, religious knights, dothraki, gladiators, experienced Nighht Watchmen, and so on. Swordsages are Braavosi fencers. Need I continue? Probably not.
Duskblades and Hexblades (except Hexblades with the errata!) are the noble scions with sorcerous blood. Some can get a level of Beguiler, too, if it fits (group votes on a case-by-case basis). You can't have more than one of those, nor can you have one, ever, unless you started with it (this can be waived after some PCs get to survive through life-changing experiences-if they didn't have one of those already. Training of the right kind can be a life-changing experience, and possible a life-ending one).
You can add the following as well: Dragonfire Adept (Dragon Magic), Dread Necromancer (Heroes of Horror) (NPC-only recommended), Totemist (Magic of Incarnum) and maaybe Wilder (Expanded Psionics Handbook).
(Originally Posted by Mike Mearls
The hexblade suffers a little because he came on the scene relatively early in 3.5's life. As R&D pushes the boundaries of the game, we learn that some things we thought were risky or potentially broken aren't. Other times, we learn things that look fine don't actually work in play. Armored mages fall into the first category. Them seem really powerful, but in the long run they aren't. Spells and magic items allow an unarmored mage to build great defenses. The spell mage armor is as good as medium armor, and its duration allows most mages to keep it active at all times. If you compare the hexblade to the duskblade from PH 2, you can see how the thinking has changed. If you want to boost the hexblade, I'd try the following changes:
Good Fortitude save
Curse ability usable 1 + the hexblade's Cha modifier per day
Curse ability usable as a swift action
Curse ability does not count as used if the target makes his saving throw
Ability to cast in light or medium armor and while carrying a light shield or buckler
At 6th level, the hexblade can cast one hexblade spell per day as a swift action, as long as its original casting time is a standard action or faster. He gains an additional use of this power at levels 8, 11, 14, and 18.)
Maybe, and that's a big maybe, Favoured soul.
Never more than one magic/psionics-using class per character, though. And never more than one in three characters can have a class that gives spellcasting levels. That rule is still in effect!
Of course, most enemies are going to be humans, and most of them are subject to the same restrictions. But then you know that already, I suspect.
Well, that's about it. Have fun and remember, a game is not only what is allowed, but also what isn't allowed