Sony has confirmed to Android Authority that the Xperia Z5 will be the last generation of the Xperia Z series, with no Xperia Z6 release planned. The new Xperia X series is still unofficially water-resistant though.
(Update: Sony confirms no Xperia Z6) Sony Xperia X to hit US, may usurp Xperia Z entirely
While Sony's new Xperia X Performance is touted as part of a brand new device line, it feels a little too much like what we would have expected the Xperia Z6 to have been if it had've shown up at MWC 2016 instead.
Sony reinvents Xperia brand with new Xperia X, X Performance and XA (updated)