NikonRumors reports that a new Nikon D6 camera is rumored to be announced in 2019.
CanonRumors reports that a Canon EOS-1D X Mark II camera replacement is expected to be announced before the 2020 Summer Olympic Games Tokyo that will start in July of 2020.
I think it is safe to assume that Sony will also replace their a9 camera for the Olympic games next year (Sony a9 Mark II?).