The second act has been planned a long time ago, but it hasn't started yet. Aegon VI has only conquered Storm's End in (hopefully the beginning of) The Winds of Winter, waiting for a confrontation with the Tyrells. Daenerys would hang around the Dothraki in the first half of that book and must first return to Mereen before she sails towards King's Landing.
The war against the Others: this was originally planned for the third book of the trilogy, after the Dance of Dragons. We know almost nothing about these mythical creatures. I believe that Brandon Stark with his supernatural powers will find out something about their culture or motives. There is nothing that hints at a fight against the Others in the short term.
We still have two books with two story acts to start. I see the (second phase of) the War of the Five Kings only end when Aegon VI conquers the Iron Throne and with the restoration of the Stark-rule. Both will happen at the earliest (although unlikely) only halfway TWOW. The Second Dance of Dragons can only begin when Daenerys lands on Westeros, after the coronation of Aegon VI. There is then only a maximum of one and a half books to cover the two upcoming story acts, which is very tight.
That worries me a lot, which means that the upcoming story acts will have to be shortened, in a very rushed timespan. Even the tempo of A Storm of Swords doesn't seem to me sufficient to complete both acts with just two books. I'm afraid that a very rushed timespan will be at the expense of worldbuilding, which is George R. R. Martin's best talent after all.
Note that in addition to the three story acts, the storylines of about twenty POV characters also need to be completed. As it stands now:
The Wall and beyond: Jon Snow was murdered by his own men as a result of violating the political neutrality of the Night Watch. This causes absolute chaos in the Night's Watch, with three warring parties.
The Baratheon camp (under Selyse) that wants to marry its own men to the wildlings (without success).
the wildlings themselves who want to leave the Night's Watch to save Mance Rayder.
Bowen Marsh and his men who simply want to put order back into the Night's Watch.
Mellissandre is the only POV currently at the wall. I have my doubts that Jon Snow will be brought back to life in her first chapter.
Brandon Stark is still in ADWD just starting his training as a greenseer, with the three-eyed crow, Brynden Rivers. I'm assuming this will largely happen off-page, except for events like "Hold the Door" (actually Hold the Pass).
The North: Stannis is still preparing to defend the Crofter's Village, where an attack by the Frey's and Manderly's is expected. Stannis is expected to win this battle (possibly according to the Night Lamp theory) and later to conquer Winterfell for the Starks. There are few other options: it is unlikely due to distance and weather conditions that Stannis will lose, march back and burn Shireen, who is now at the wall. This will only happen later in other circumstances. But the whole Stark-restoration does have complications:
Davos Seaworth will spend some chapters in Skagos, where he will probably find Rickon Stark, who may become the Lord of Winterfell. However, one must also take into account the time of the outward and return journey.
On the other hand, we have Jon Snow (who lies dead on the Wall in the present), who can somehow be legitimized with Robb's Will and thus be crowned King of the North. Here, too, the distance has to be taken into account. And nobody knows how Robb's Will will be made known and when.
This whole succession of the North will not happen without problems, there will probably be a succession conflict between Jon Snow and Rickon Stark, a conflict with two possibilities: independence or service and subordination to King Stannis. Sansa might also want to make a claim.
The Vale: Sansa Stark may or may not be married off to Harry Hardyng. Petyr Baelish will also get into trouble with his rule over the Vale and the Riverlands (he doesn't have an army). I don't expect Sansa to stay in the Vale either.
The Riverlands: Brienne and Jaime will be judged by Catelyn Tully, now known as Lady Stoneheart. What will happen to them is still rather unclear. There are hints that point to a second Red Wedding and a fairly important role for Nymeria, Arya's wolf.
Crownlands/Westerlands/Reach: We will see the decline of Cersei after she somehow regains the role of regent. Because they are surrounded by the throne pretenders Euron and Aegon VI, it will be an enormous chaos. Expect the preparation for these wars, a reaction to Kevan's murder, a confrontation with the High Sparrow, the return of Myrcella Baratheon and the fact that Nymeria Sand will participate in the small council. According to George R. R. Martin, we will also see Highgarden and Casterly Rock in TWOW. We may also meet Willas Tyrell in one of the two books, as once said - the Tyrells will continue to be a significant player.
Euron and House Hightower: Euron is now at war to conquer the Reach, which I think will be mostly off-screen (some will be viewed from the perspective of Samwell Tarly or Aeron Greyjoy). As far as Samwell is concerned, he is still in the process of completing his training to become a maester, although I have my doubts that he will complete it.
Dorne: according to George R. R. Martin, Dorne will play a major role in TWOW, which includes the Sandsnakes in King's Landing. It is very likely that Dorne will form an alliance with Aegon VI Targaryen. Quentyn's terrible death may also be made known to Doran Martell. There is also still a hunt for Gerold Dayne
Aegon VI and the Stormlands: According to Arianne's two POV chapters, Storm's End has been conquered (although George R. R. Martin also plans to dedicate some POV chapters to the battle - which was originally off-screen. Also expect a decisive battle that will allow Aegon VI to conquer the Iron Throne without too much trouble, for example a kind of Battle of Agincourt against the Tyrells (as Bryndenbfish describes in his essays Blood of the Conqueror). Aegon VI Targaryen will have to sit on the Iron Throne before the Second Dance of Dragon starts.
Braavos: George R. R. Martin has already written many chapters for Arya in Braavos, but I have my doubts that she will complete her training to become a Faceless Man. The first opportunity for Arya to return to Westeros is through Justin Massey, Stannis' advisor who wants to hire a mercenary army with a loan. He will only leave after Theon's first chapter in the Winds of Winter. Taking into account the distance it will take some time before he arrives in Braavos.
Meereen/Daenerys/Tyrion: Barristan Selmy and later Victarion Greyjoy get involved in the Second Siege of Meereen, which will take place in the beginning of The Winds of Winter. I assume that these will take up quite a few chapters. Tyrion will also join the court of Meereen. Meanwhile, Daenerys will be in the Dothraki Sea (perhaps as a prisoner). It will take a long time before Daenerys and Tyrion will meet (so expect a reasonable late return). Moreover, things have to be sorted out in Meereen before Daenerys travels to Westeros with the help of Victarion's fleet. Moreover, it is possible that Braavos (the House with the Red Door) and Pentos (the Tattered Prince) will also play a role in Daenerys' storyline.
You can see how many storylines still have to be completed before act two, the Second Dance of Dragons can take place... A lot happened in A Storm of Swords, I know that, but I still have my skepticism that George R. R. Martin can complete all these storylines even at that pace. I wouldn't be surprised if The Winds of Winter were about the same size as A Feast of Crows and A Dance with Dragons together,
If I was GRRM I would just simply drop most of those plotlines in the next couple books. Skip them entirely. Cut down to the main theme and plot and get through the slog before age and weariness prevent me from the finish line. Once that's done, he could always go back and flesh them out in additional books and novellas.
But clearly that's not what GRRM is doing, or else we'd have a new book by now. I personally doubt we ever will.