I disagree about the iPhone S6+ needing more than 1080p. It's not a deal-breaker, and people would only complain that the battery life has gotten worse!
I wouldn't mind seeing the iPhone 6s getting a 1080p screen though to appease the masses, but I also wouldn't mind it staying the same, since 1080p on a 4.7" screen is a bit overkill.
But both of those go hand in hand. I would love to see a 2K display on the IP7 and get great battery life. I would also say that in today's smartphone world a 2K display is the norm now. I would also like to see Apple optimise their technology so it will still get great battery life like my 6s+ does.
There's an easy way to achieve both: Make the phone thicker. In reality, Apple isn't going to release an 8.x mm iPhone 7, so I think better battery life is a priority.
Why is 2k the norm?
Ask yourself how much PPI you really want your screen to have. Currently, the Plus has 400 ppi, and to me, 350-400 is roughly the magic number beyond which I stop giving a poop about resolution. Not sure about everyone else, but Apple only has 1 phone model with 1080p, and yet the lower res phones still sell. Even the 5S sells well, and the SE will as well. The iPhone 6 and 6S have obviously sold well, and they're closer to 720p phones.