Everyone's laughing about this letter- and that included myself- but I really wanted to break down aspects of this letter with my opinions.
As much as we can get a good laugh, much of the letter has remained true in the story, albeit sometimes revamped. For instance, the whole Lannister/Stark conflict with Dany the last Targaryen but the real foe is the Others has remained in tact. While Theon/Ramsay and not Tyrion burns and take Winterfell, it does happen in the books. Robb, Ned, and Catelyn all die as well in this draft. Bran has magic, goes Beyond the Wall and is a big character. Arya fights with Needle. Viserys dies because of the Dothraki and Dany still thinks about that in ADWD. Dany is planning on invading Westeros. Jon becomes Lord Commander. All of that stays the same. Even the mention of Jon's parentage with R+L=J seems consistent.
King Jaime: The Jaime bits make sense for the Jaime we're introduced to in AGoT- the one who tried to kill a child of seven and the one that Ned describes as sitting on top the Iron Throne when he arrived. I feel like it's believable that GRRM gave him more depth later- he did so with a lot of non-original POV characters so far as I can tell. And we really do get a shallow view on Jaime in book 1.
Our Jaime doesn't crave the throne but the initial conception of Jaime? Yeah, you could see that set up in AGoT. And the Lannister conflict as well as the power grab after Joffrey's death in the Red Keep is consistent.
Sansa: The Sansa stuff is arguably the most believable though because Sansa was the main character who was likely to be least fleshed out by this point seeing as it's super early and we know she was late to be conceived by GRRM. Moreover, I've always felt GRRM wasn't sure where he wanted to go with Sansa in AGoT. ACoK onwards is where I feel she really becomes more complex and well developed.
Also, there was a conflict in which Sansa "chose" Joffrey/the Lannisters over her family (because she wanted to marry him.) And I think we can all agree that she regretted it and wished she could leave Joffrey and court.
GRRM claimed that he created Sansa to add conflict to the too-happy Stark family. That's pretty obvious in this version of events. This is more of a one-dimensional and water down version of the Sansa we know and love.
Bran: He does wake up from his coma with prophetic dreams. He does flee a burning Winterfell and head Beyond the Wall. He's also had limited contact with Jon/the Night's Watch.
Arya: Arya does flee the Red Keep aiming for Winterfell with an adult (Yoren) directing her. She has Needle and Nymeria's still alive. Moreover, she has a lot of contact and connections with the Night's Watch even with the changes. Also, she does try to get to the Wall even in Braavos many times to get protection from Jon.
Tyrion: Tyrion does become disenchanted with his family, he does form a bond with Sansa, he does have a family conflict that forces him to flee King's Landing as a fugitive- all of that is true.
Tyrion/Arya: It's possible that GRRM changed that to Sansa/Tyrion because Tyrion does have attraction to Sansa that she doesn't return.
Arya/Jon: Now, unpopular opinion time, but I can actually see this with GRRM. Personally, I don't ship it, but it as arguably be seen as set up within the series. Arya and Jon's connection is the strongest bond in the series. I don't see how people can claim otherwise whether it's platonic or romantic, they think of each other more than anyone else does.
And the whole idea of love for Arya vs. duty to the Watch was a major plot point in ADWD. "Different roads sometimes lead to the same castle." There really is foundation if GRRM wanted to say it was romantic and not platonic. Given GRRM and incest, it's not surprising to me.
When I read about Jon/Arya being canon, I wasn't surprised. If they weren't related, readers would think they were madly in love given how much they mean to each and how often they think of each other. Arya "could always make Jon smile", "She missed Jon" more than anyone, even just the idea that Jon specifically wouldn't recognize her in ADWD makes her depressed.
As for the five most important characters (Arya, Dany, Tyrion, Bran, and Jon,) no one should be surprised. I don't think people can consider them safe, but they've always been set up to be big players against the Others and the only original POVs left.
But what about Sansa? Well, like I said, I don't think she'd been fully conceived yet so her absence means nothing one way or the other to me.
That's my take on everything. Take it with a grain of salt, but I wouldn't discount everything. Mostly I'm looking at this as interesting in terms of how it all began and how much has changed.