I think the successor to the Ultra will be the rumored "Z3 Tablet Compact" - i.e. a device marketed as a tablet rather than smartphone. I am sure Sony will have more success that way.
The Z3X seems to be more of a camera centric device rather than a direct Ultra successor. Because of this, I consider the Z3 Tablet Compact to be the replacement and it would work, especially with a 7 inch screen. I don't think they will make an 8 incher considering that it would be close to the Z2 Tablet.
I would not like to go for a smaller screen on the next device and the camera part is covered through a QX10 - I would most probably opt for the tablet instead. I would be quite sure that it will have full phone functionality even if they push for the pure tablet related aspects in their marketing communication.
[Xperia Z Ultra C6802][ROM: GPe 4.4.4][Launcher: Nova][240 DPI]
[Xperia Z C6602][ROM: PAC 4.3][Launcher: Nova][240 DPI]