The women are packed from top 10 all the way to 50. That is why I suggested a mix of players making names for themselves in slams next year. (and of course I hope Hsieh scares some people in 2019) I am not too optimistic on Serena, Kerber and Osaka in slams in 2019.
The men though are in a broken stratum as it appears to me, the giants were too dominant for too long so nothing else could grow. While falling into decline, there is complete drop off in the link between them and the next level. This is the opposite of the WTA. The current ATP reminds me a bit of the WTA just after the mid eighties. The problem is that there is no new thought or innovative thinking in the men's game anymore. A sudden shift at some point, maybe not next year or possibly the next, could result in somebody, even deeply flawed compared to the previous 3 Kings, taking all the slams almost by default.
I hope Djokovic holds the line for a little longer.