We're told that the follow-up to the Canon EOS-1D X Mark II has begun field testing with a very small number of photographers and/or engineers. The Canon EOS-1D X Mark II was announced on February 1, 2016. This makes an imminent announcement seem highly unlikely.
2020 is an Olympic year and it just so happens that the summer games are in Tokyo, Japan and Canon is a huge Olympic sponsor, and will likely be an even bigger one with the games being in their home country.
We have no idea what the specifications for the camera will be at this time, and we don't expect an announcement until the very end of 2019 at the earliest. An announcement ahead of CP+ 2020 next February wouldn't surprise us.
Whenever it does come, will this be the last of the Canon EOS-1D series of DSLRs?