TENAA has certified the upcoming Sony Xperia Z3 and we now have a virtually complete specs sheet. And we still find very little upgrades over the Xperia Z2 model.
The phone keeps the 5.2" IPS LCD with 1080p resolution but is marginally smaller. The Xperia Z3 measures 146.5 x 72.4 x 7.4 mm while the current Xperia Z2 is 146.8 x 73.3 x 8.2 mm. It's a hair lighter too (158g vs. 163g) but it was always the bezels that drew complaints and they have barely changed.

The chipset has been bumped up slightly to the AC version of Snapdragon 801 (with 2.5GHz CPU instead of 2.3GHz). The phone still has 3GB RAM.
The camera is the same too, Sony's 20.7MP camera will be back for a third time. The front-facing camera is a 2MP / 1080p shooter. The phone has 16GB built-in storage and a microSD card slot.
There will be a new color option, Amber, which seems to replace the usual Purple version. The product photos of the Xperia Z2 don't really show a new design either.
Note that two versions were certified – one with China-specific mobile connectivity and one with standard GSM/LTE.

Unfortunately the TENAA specs don’t list the IP rating though there's not much room for improvement there – the Xperia Z2 already has IP58, the company can at best slightly improve the dust resistance by moving to IP68 (IP5X means dust might cause cosmetic damage).