I have read all 5 books.
I have stayed away from the snippets on Martin's "not a blog". (Not really. I lied about that.)
I have read far too many "theory" sites.
However, the show seems to be paying all but lip service to them. Let's just say that they are … guidelines.
The following are what I THINK MIGHT happen on the HBO show. They are governed a great deal by "what I HOPE will happen".
In no particular order of importance.
1) Arya kills Ser Merryn Trant. She is training as an assassin, in Braavos. Cersei is sending Lord Buffoon Tyrell, Master of coin, to pull the Donald Trump Too Big to Fail routine on the Iron Bank of Braavos. Good luck with that, they are already backing Stannis. Anyway, Cersei is sending Ser Merryn as his Bodyguard. He is on Arya's shortened hit list. Do the math. Predictable, but high on my wish list too.
2) Jon will have his Ides of March moment, like in Book 5, with Olly as Brutus. However, Kit Harrington is signed up for two more seasons, so never fear!
3) Theon will man up. He did in the books. Nice.
4) Catelyn will be avenged, Brienne will give the Stark's regards to Roose Bolton. (Ramsay is too much of a loose cannon for me to predict.)
Come on. Don't you want to hear "Lady Stark sends her regards!" Sure you do. She can die heroically, and what a way to go as Sansa and Theon escape in the chaos. Sansa is good at escaping in the chaos.
5) Lady Stoneheart will start killing Freys. This has been an off and on tease for two years now. I think it will happen, and Fat Walda will be victim #1. It would be nice for it to be on Walder Rivers who slit her throat.
This one I am least sure of.
6) Jon snow's true parentage will be sensed by Mel, but not revealed. Her eyes will grow wide with shock, and her knockers will be on fine display. The true reveal will be next season.
7) This will be next season too. Dragon glass killed one White Walker. Dragon (Valyrian) steel killed another. Why not real Dragons?
8) Stannis will bend the knee to … Dany. In the books, he was very reluctant to side with his bro in Robert's Rebellion. It conflicted with his Duty. But he came around. Like Frederick in Pirates of Penzance, his Duty will tell him that since Dany is a True Targ, he should follow her. After all, 300 years and 20 generations, usurpers can be legitimized. The Baratheons have not had nearly enough time.
This one I am even less sure of.
9) "The Dragon has three heads"
They will be Dany, Jon and Tyrion.
Dany is obvious.
Jon's Ides of March will mean his Watch is ended. R + L = J and all that.
The Mad King got jiggy with Tyrion's mum, Tywin tried unsuccessfully to abort the baby, the fetus was damaged… Tyrion!
10) Things will get out of hand in the fighting pits. Drogon will swoop down and rescue Dany.
Someone leaves the doors unlocked, letting the other dragons out.
One swoops down and carries off Tyrion, much to his delight/dismay. But they bond.
The last one flies north to seek out our Ides of March victim.
11) Samwell becomes the new Lord Commander, and he annuls that celibacy part of the vow, making it optional.
He sends Dad a selfie.
Fill in your pwn speculation regarding Cersei, Jamie, Littlefinger, Gendry (Still rowin'!), etc.