I really love the back and forth... Rumors of 2800x is they are waiting for the 8 core Intel release. I read that the z370 might not be compatible and like you have posted, the z390 or something would be needed. Possibly the 2800x will be 10 core since the clocks are not going to be that high.
They can't do a 10... they didn't redesign the arch, just put it on a new process and made some minor tweaks. They could do a 10 core on the X399 platform I suppose but that would put it outside the price bracket of the mainstream.
I'm not sure there will be a 2800X in the X470 platform that we know.. and if there is... if it will be any better than the 2700X
Zen 2 will have to be the next counterpoint.