Here's a potentially interesting thought for next year. 
Since their seems to be a wide disconnect between people looking for value vs those looking for premium,  what if Google just made nexus into its own proper brand and offered 3 models next year. 
Nexus 4 $200,$250, 5 inch screen
NEXUS 5 $400,$450, 5.5 inch screen 
NEXUS 6 $600,$650 6 inch screen 
Not sure about the specs other than the highest end nexus 6, which would have metal construction,  4 gb ddr4 ram,  820 processor, front facing speakers,  23 mp camera,  64/128 gb memory,  and 4,000 mAh battery. 
Trying to be realistic about what's possible not what I want. 
For example if love a 1080 display to double screen on Time,  but that's never gonna happen,  not with everyone else packing 2k and now Sony. Packing a 4k in the z5 premium. 
If also love a thicker phone,  9 mm,  nice hey to it and capable of a 4500 or even 5000 mAh battery. 
Anyway,  I think a low,  mid,  and high end nexus would really help build the brand and let me people experience the joys of stock Android. 
Another interesting thought, since android phones are now in a race to the bottom on price because no manufacturer seems willing to take the effort to cultivate a brand and fan base (moto was doing it until Lenovo fired the people behind all the moto x features we loved) how about nexus take a stand and try to stand out by offering more than just stock Android. 
I'm thinking: 3 year pledge for updates. Meaning the version the phone comes with plus the next 2 as well. 
That's longer than any other manufacturer is doing. 
Also, big batteries,  kinda how Droid maxx was all about the 2 day battery,  let's to set a new standard for battery life,  2.5 days for the smaller phones. 3 days for the nexus 6 phablet. 
Oh and finally,  duel facing speakers because once you experience them going back sucks. 
I'm tempted to also say water proof design,  so basically we're having Sony build them, but I don't want to get too greedy,  but what the heck,  ip68 it is. 
If I'm paying $650 for a phone I want it all