The hound and Jamie probably have the most significant stories from the past that via the book....and are some of the few that are still around from the beginning so no shock.
I got to thinking if they had got to the point of needing to wait for a book, they could just go backward and do a season on the previous king (Ned Stark, Jamie, Robert, king aegon aka daenarys dad and dragons would be central characters). Would be cool. There's a whole war that's only touched on through dialogue from the show
there are a few characters who are still around from the beginning but the hound and Jamie have better character development
I read people say that they are only doing 7 seasons. If that's the case then I don't know why they would split a book up into two seasons?
btw, I just realized something. Cersei is pregnant. that's just a guess but she's either pregnant or going through menopause or had an abortion or something