Well, I will put my two worthless cents in as well. What do I know? I am just guessing anyway.
Like Dennis, I think a D6 will be in the works and this could very well be the last DSLR. I think they will up the MP on it and the buffer and the FPS rate so your jaw drops. And, you will faint at the price as well.
I do believe they will introduce a Z camera like a 9, in the coming months. Like Dennis, I think the 2020 Olympics is key for marketing and optics in the industry, so they have to be there. Maybe 60mp, maybe not. I don't think this is necessary anymore. The MP war, in my opinion, is over. The files are simply too big now. So, who cares.
I do believe things like eye recognition, AF, FPS, big buffers, big batteries, and nice grips are extremely important in the next wave of Z cameras. Some upgrades to in-camera image stabilization are also expected. I think it still needs some worth with really long lenses.
I think we might hear something by the end of the year simply because they never deliver anything when they announce. So, a 2019 announcement late in the year, and delivery to Nikon Professionals in time for the Olympics in 2020.
A comment on Nikon just jumping into the game seriously..... Hi-Tech is a very fickle industry. Just because you are first and you have the lead, doesn't mean much. It all comes down to how determined and how rich your competitor is. If Nikon is resolute enough to make the big investment, and I believe they are, they will be back in the game in a serious way. I like the Z line. In fact, I am very impressed by it.
Having used the gear for almost a week, I am very impressed by the feel and performance. Their road map is solid and I believe they will do well with it baring some stupid decisions we can't anticipate.
I also agree with Dennis that 2020 will be all about sports, so everything to capture fast-moving subjects will be the focus of the industry/Nikon at the very least.