He posted this in his Not a Blog.
He said that we will have one or maybe two books in 2018, but knowing him that is wishful thinking at best. I am pretty sure (and I really want to be wrong) that we will have one book in 2018 but it will be the Targaryen shit and not WoW.
Meereenese knot 2.0 is a tough one to crack
Though I don't believe he ever solved it in the first place, since the show has revealed that the Crux of the knot is the yet to be achieved timing of dany returning with dothraki to win the battle of meereen. In other words he just pushed the central problem to the next book. Remember, the first dany chapter of AdWD was supposed to be the fighting pits where she flies off on drogon, and then she spends the rest of the book subduing the dothraki and returning to meereen. And whilst in this stage of writing before AFFC had been created, Martin was already talking about the meereenese knot.
Once he eliminated the five year gap, he started writing dany chapters to preface her fleeing the city, and each of those chapters further threw off the structure of the book, because other characters now needed more fleshed out journies so dany had time to be motivated to flee meereen. So rather than one prologue chapter introducing Quentin, one introducing Victarian, we get three of each as they go from a to b to c.
Then we get further Tyrian chapters to lengthen his journey to match and then further dorne and iron islands and blackfyre chapters to further flesh out the supporting story for the new storylines, and dany gets more chapters in meereen leading to a snowball effect pushing the central problem of convergence with dothraki to be even more impossible.
Unfortunately, by the point the book has snowballed into this unwieldy mess they made the disastrous decision to split out AFFC. This was a long term disaster because it locked In those chapters. I imagine without it being published he would have rewritten the bulk of it (and much of ADWD ) to be more compact, but instead all of these loose ideas of which character would work as the best pov got made into permanent choices. So rather than Martin deciding between arys oakheart, Arianne, or areo, his perspective options he was exploring were made into permanent selections, and rather than collapse six chapters into three we got six. Same with iron islands.
What got it in publishing shape was using Barristan at the end of the book to unify the various loose plot strands in meereen that all arrived too early. At the very least this created some dramatic resolution