What if Apple goes for broke: 6" iPhone?
It seems the running desire for an iPhone screen size change is 4.5", which I agree would be ideal. However - what does it add, really? It would be a 1280x720 screen at the same DPI, which does seem more like an attempt to claim certain numbers. We're ultimately talking about a screen without appreciably greater real estate, but enough that it would affect development, since many developers still eschew autolayout in favour of building interfaces for each individual screen size.
We currently have an iPad mini and iPad Air that is now purely a choice of screen size - they both have the same SoC and much more proportionate weights and sizes, not entirely unlike the screen size choices in the MacBook Air and Pro. My theory, is that if Apple wants to go bigger with the iPhone, they'll do something similar.
My bet, they keep the 4", with A8 or whatever it's called, and introduce a 6" model to fall in between the 7.9" iPad mini (perhaps take the opportunity for a WiFi-only "iPad nano"). Keeping the same DPI with just broader screen real estate (so assets remain the same physical size), that's a 1706x960 screen, a 125% increase in real estate (contrast 27% with 4.5"). Assuming the SoC doesn't need to be significantly physically bigger (the GPU would obviously need to run a bit hotter to handle all the extra pixels, though considering this is still smaller than the iPad resolution which the same GPU powers now between iPhone and iPad, this shouldn't be much of an issue), the battery would have 2.5x the capacity (contrast 1.5x with 4.5"). Besides the GPU running a bit hotter, it would also need 50% more backlight power.
I don't know if I would actually get one - it's important to distinguish, when predicting products, between what you _want_ and what they actually might _do_ - but it's become clear that "phablet" is more than a niche, and I could foresee not needing an iPad mini with one. Note that a 6" iPhone would indeed be slightly taller than the iPad mini is wide, making for a very comfortable typing surface. It would also be equally wide as the 3.5" iPhone is tall, so imagine the comfort of typing in landscape mode with the real estate of portrait mode.