What if:
Apple increased the screen size of the next iPhone.
Apple used increased space from the screen size change to implement a larger Battery.
Apple did something to improve the Batteries they use overall.
AND they implemented IGZO or a similar tech to further improved battery life, maybe even similar technology for bluetooth/wifi, things that use up the most Battery in general?
That alone IMO would give me enough reason to upgrade to the next iPhone from my current 5s, depending on how much of a difference was made. Which would probably be a large gain, considering they would be improving three aspects of Battery usage.
I always have brightness below 50%, and always have Bluetooth off and I still wish I had more battery life. Not A lot more, but imagine if Battery Life was improved 20% maybe even more? Usually my phone will last most of the day, but on days with heavy usage and not much free time to charge my phone, it becomes a legitimate problem.