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Google Nexus 6
The Nexus 5 is not that old, but Google is on a yearly cycle like Apple so it is likely we will see a Nexus 6 in time for the holidays. CNET posted a story about an Android account tweet that had the rumor mills stirred up. Google I/O is coming up in a couple of weeks, but I highly doubt any Nexus hardware will be revealed with the focus on Android Wear.
The Nexus models don't always have the best specs, but Google did a great job with the Nexus 5 and it was a competitive device that sold for a very low full price. I can't find any information on the specs, but if we do see one it is likely to have the 805 processor, 3GB RAM, and an improved camera.
There is also talk that we will not even see another Nexus device, but Android Silver in early 2015. Maybe we will get some indication of the Nexus future at Google I/O.
Sony Experia Z3
I bought a Sony Xperia Z2 last month and just sold it a couple of days ago on Swappa. The hardware was fantastic and I already am having sellers regret, but the camera did not meet my expectations and the all glass construction worried me a bit.
The Z2 launched just six months after the Z1, but the display was a major improvement and was worth getting an update. The rumored Z3 may come in September, about six months after the Z2. It may not yet have the upcoming Snapdragon 805, but Sony may work on reducing the rather large bezel space around the display.
The Z2 is already quite thin, but Sony may be reducing the thickness even further. We don't know much else about the Z3, but you can expect Sony will continue with waterproof designs with lots of doors.