Here is my prediction. 1DX3 in spring of 2020 for the Olympics. 1D equivalent in beta test for olympics to tweak software for fall 2020 release.
I have a 1DX2 and I can see room for some improvement which I think Canon will do for the 1DX3. USB-C, 4K 10bit output via HDMI, bump to 22-24mp (likely 24 since the a9 is 24), clog. I would love to see built in wifi but I do know the reasons why its on a separate dongle. Would also love to see both slots to be the same card. I don't really see a need for more FPS but its always an arms race so I would probably expect a 2fps increase. I just don't see Canon having the tech to put all of this in a mirrorless camera just yet unless they have an ace up their sleeve.
What about a 5D IV replacement? When do you expect that and what kind of feature?