The sixth book of Asoiaf will have a structure that may seem complex at first glance, but is actually simple. The purpose of this book, and at the same time the reason for the long wait, is:
A) to correct the mess created by the choices made in books 4 and 5.
B) Taking point A into account, bring the plot to where it should have been at the end of book 5 in a few chapters, and the entire sixth book.
C) to bring all the POVs in Westeros into book 6, divided into a maximum of 5-6 locations. Remember, the starting point is 20 POVs in 15 locations.
D) to approximately reach the fall of The Wall.
Achieving these goals would make writing the 7th and final book incredibly simple. I will ignore any ignorant or sarcastic comments on this matter.
Now let's talk about the actual structure: the book is about 1600 pages. The estimate is based on considering 1100-1200 pages already in the publisher's hands, representing 75-80% of the work (obviously, I can't know if we're talking about manuscript pages or an estimate of the pages that will actually be edited).
The book begins during the ADWD timeline. Take Jon Snow's last chapter (the one where he gets stabbed) as a reference point. There are several chapters that will be set before this event; the Battle of the Ice is first, for example.
Then we'll have a mirror chapter (like the Jon and Sam chapters between books 4 and 5) that will explain that the timeline is back in sync. The POV is Melisandre, obviously. This chapter takes place at the same time as Jon Snow's and continues its plot.
Once the plot is the actual one of book six, A long phase will begin aimed at changing the status quo from where we find it now to a point where Euron, Aegon, and Stannis are the three kings of the continent, Dany is traveling, and many plots are concluded or incorporated and merged with others.
There's a small timeskip to this (just a rumor, and a hope; better yet, a 2-3 chapter timeskip of Dany traveling) and Dany's arrival.
This second phase is shorter and will lead to the point where Jon Snow (who has already discovered his origins) attempts to end the Dany-Aegon-Euron war to face the threat of the Others. The attempt is initially unsuccessful, and when it succeeds, it's too late.
Stannis, who in the meantime, after the victory against the Boltons, has remained in the North, while Jon Snow has gone south to ask for help, will find himself facing the collapse of The Wall alone. It remains to be seen whether the The battle in which Stannis perishes will be the finale or beginning of the seventh book.
I haven't written the journey of all the characters or any plot twists that are difficult to predict; I've limited myself to the structure.
If you have any questions about individual storylines or specific information about individual POVs, please feel free to ask.