I'm kinda interested. I generally prefer to avoid anything where you have actual political hierarchies between PCs, but in Westeros everyone's so murderously insubordinate all the time I'd guess anybody who wants a high position'll need actual talent at diplomacy to keep it. I'd personally suggest NPCing everyone above the status of Lord of a minor House or equivalent, but that's personal taste, and I'm sure there's a lot of folks who would like to be king.
It's still worth stating that this is a potentially very volatile setup; you have power dynamics between players and also groups set up for a PVP conflict in a setting that is rife with treachery, assassination, and other areas of play that have the potential to make for great storytelling in novels and truly malevolent OOC viciousness in roleplaying games.
At some point, some person whose backstory is "best warrior in the Warrior's Sons" is going to try to kill some person whose background is "best warrior in the King's contingent" and some other person whose background is "leader of a band of insane amoral assassin-clown-ninjas" is going to declare they've both been poisoned with the Tears of Lys already and some fourth person, in the background, will seek the ancient spell for turning into a huge-ass dragon while pointedly ignoring other characters except for sex. These are all things you should have a plan for in advance.
I'd definitely make your conflict resolution system eminently clear up front (I am personally fond of the write-off: X posts each, GM decides who wrote it better, victor gets last post and loser suffers some penalty - death, "victor's choice," or maybe a GM-mandated penalty limit that turns into unlimited "victor's choice" after you trip it the third time or something), including how you intend to handle stealth and deception in a freeform setting. There are a lot of ways to handle a couple folks with swords, but it's trickier if your character concept is face-changing vizier and what you actually want to do is write other characters out in ways they can't react to. Honestly if you could limit direct action between factions that would probably be for the best, but if people think they CAN go for the throat, there's a good chance they will.
Define as sharply as you can the limits of magic and PC access to magic; whatever edge you pick it's likely someone will want to straddle it, so pick an edge you can live with. I'd recommend being super-clear and up-front about exactly what the stakes are for PCs and what happens when they pay those stakes - if they die, are they out of the game, do they get to make another character? If they can't die, where's the line on how far the world bends to keep them playing and where's the line for what CAN happen? A black cell might be as good as death if it means you can't do anything for long enough to lose interest in the game, for example. Some people aren't gonna want to play with even "I'm taking your Valyrian steel sword" on the table in terms of consequence, whereas some people will turn away if they can't rape and eat each other with reckless abandon. Decide who you want your players to be and pitch hard to that audience. Make your boundaries clear, to continue the theme.
