Bigger battery, though not massively bigger. Around 3,000 mA would do fine seeing as a good chuck of the battery issues cropping up are more Google software related. For some people, like me, it's great and for some it's awful. Bigger battery mitigates that for people having issues, but also gives Google a free pass not to get their OS in shape. It sucks either way you look at it. I don't care about thinkness and I like the camera hump. Otherwise it's a plain, flat slab that looks very iPhone-ish.
Software is the next big one, if not number 1 for me. The backend of TouchWiz got an overhaul and works much better but the UI still has issues. Several of the apps, like the dialer app and calender app are well done and have a decent enough material design touch but some of the other apps look like fancy Jelly Bean (or earlier!) apps. The TouchWiz apps need to have a unified look and 99% of the icons need a full redesign to fit more with material design.
Bloatware will never go away. Samsung really cut it down with the S Apps but the carriers went back and mucked it up.
Speaker needs an overhaul. Bad. Front facing would be better but even if they keep it at the bottom it needs to be of much better quality. The thing pretty much screeches out it's own ringtones when it's set to full volume, and does much worse with any other audio that could come out.
Water resistance would be nice, and should be fairly easy on a sealed phone, but I would not call it a biggie.
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