Ah, well if you've finished the 3rd book you're just getting into the good stuff. The 4th book was decent enough to make me impatient as hell for the 5th, but the 3rd has remained my favorite so far.
My line of thinking was more about the Lannisters outlook towards the approaching winter, and how they don't really seem to give much heed to it. The Starks (while they were around anyways) were much more about planning ahead and being prepared, since "Winter is coming." So for the Lannisters I was gonna do a wheat brew, something summery that you could imagine lots of being drank. I've even got an idea for some manner of oaked ale for the Night's Watch.
Although, this idea of a brutal, sneaky beer does make me wonder how I could pull this off...but being a newbie, I don't know if I've got enough skill to pull it off, lol. This is tempering my ideas, I don't want to take on anything I can't really do yet.
So far, the lineup is coming up as such, and will probably take up most of my brewing efforts for a while:
Stark - Something stout, a more "wintery" beer
Lannister - A weizenbock or other wheat brew...though I really love weizenbock
Night's Watch - An oaked ale of some kind
A cider of some sort, like the kind made in Old Town
Probably a mead too
Those last two are really because I wanna learn to brew em, but they fit with the theme. I'm trying to think of other brews that fit well within it as well...