This is similar to the situation GRRM encountered whilst writing the Battle of the Blackwater at the end of A CLASH OF KINGS. He had a huge, complex battle to depict involving three POV characters (Tyrion, Sansa and Davos) and numerous major non-POV characters (Cersei, Sandor, Pod, Meryn Trant, Stannis, Ilyn Payne etc). He also had a big problem in that the end of the battle, where Tywin and Mace Tyrell's armies combine and smash Stannis' flanks, was difficult to depict because he didn't have any POVs anywhere near Stannis. So writing the sequence so that it flowed well, was interesting to read and so on was quite tricky, and apparently he bounced off it several times before coming up with the ending we see in the book now.

He has similar issues with the Red Wedding, where he had Cat and Arya as POVs and lots of other non-POVs whose stories needed covering, and he had to approach it from several different angles (Cat's chapter which is the pivotal moment of the sequence was actually written after the rest of ASoS was completed).

So the Meereenese Knot is likely a similar situation arising in the city of Meereen, where Dany's story is taking place. At the end of ADWD at least three POVs, maybe more, are in Dany's vicinity. We know from AFFC that Dany and her forces in Meereen are being joined by other characters, such as Quentyn Martell, Maester Marwyn and Victarion Greyjoy, and there are also many other factors/forces opposed to Dany gathering against her. Whether that culminates in a huge battle or not (although that is quite possible) is unclear, but certainly it's a huge 'convergence', to borrow an Erikson term, where lots of characters and events come together and events have to pan out in a certain way. If it is the culmination of the book, then how it pans out will also affect the start of Book 6 (as the Blackwater also determined how Book 3 would start).

The good news is that the Meereenese Knot comprises the last 10% or so of the book, and already exists (at least partially) in a draft form. The other 90% has been sent off, edited and 'locked' by the editors and publisher. When he cuts through the knot and revolves it satisfactorily, ADWD will (touch wood) be finished.