Eh. As a world, Westeros ain't all that interesting. What makes it great is the story, and thats impossible to transfer to the mod. Any TC worth its salt would have to write its own story making the player the hero, and writing a story as good as Martin's is no easy task. Game mechanics like large armies and sieges would also be difficult, if not impossible, to do.
I agree with what you say, but for the "impossible" part. I would say, rather, "very difficult indeed".

Because it
could be done. Total conversions have happened, with their own storylines, etc.
As for large armies, I don't think most characters would need to be involved directly, so you could - if anything at all - simply use video for that. Sieges? Likewise, I suppose.
It would be one hell of a lot of work -well, I can only imagine, really. But I suspect it
could be done. Just not sure if the Gamebryo engine would be the best choice anyway... :unsure2: