Mostly depends of Novak's shape. If he will be in a good or very good shape, it will be something like that:
AO: Djokovic
RG: Nadal
WB: Djokovic
USO: Zverev
If not, then something like that:
AO: Federer or Djokovic or... Zverev? maybe?
RG: Nadal
WB: Federer or Nadal
USO: Zverev
As I remember, Murray was counted as "The Big Four" member, even before he won his first maijor. And who knows, if he would won any if he woulnd't be coached by Ivan Lendl. During that time, he won his first two maijors. After Lendl left Andy's team, Murray wasn't able to win any single maijor. He did that, after Lendl returned to his time, and even became a #1 in ATP ranking. Now Lendl's coaching Zverev, and a few months after he joined his team, Sasha won his first big tournament - 2018 ATP Finals. I bet, that he will takes his first maijor this year. AO? I guess it's too soon. Perhaps, if he will somehow be able to keep his good shape... but I doubt it. Even if, AO might be too hard.
Will Roger be able to win another Grand Slam title? Depends. Roger indeed might be called GOAT at this very moment. Along with Rafa and Novak, all three of them in their prime, they are on very different level then the others (I'm speaking of their era only). Due to advanced age, Federer past his prime and will never be as good as he used to be. But it's not necessary at all. On his moderate or lower level he still might be superior to a great majority of active players. Only Novak in a past few years, perhaps still, more likely still, is able to barely catch his prime, and be above Roger. If not Novak, Roger would probably have 3 more GS titles only in 2015-16, let alone 2011-2014. So, if Novak wont be able to play on his highest level, Roger might take either, AO, WB or USO title.
I t will be the last one, or one of two last GS titles for Rafa. I doubt that he will return to #1 as well.
Neither Roger or Rafa will anounce retriement this year. If any of them will have a some serious inuje, I bet they both will play untill 2022, I bet Roger will anounce this in 2020 or 2021. Rafa might retired ealier, maybe. Maybe 2021? Anyway, I still bet it will be '22 for both.
Murray will be the first one to anounce his retirement. I feel that he will do that either at the end of this season or at the beginning of another.
It also might be the last one, or one of two last seasons, that Novak will be able to dominate.