I would make three models and dismiss the current lineup:
- iPhone 6: 4.5" 1280x720: 320 ppi, same ppi as current iPhone, new "optimal size for productivity and comfortable one-handed usage" (ever heard somebody with a Moto G complain for one-handed usage? Me neither), also effortlessly compatible with old apps (just a little bezel is added from 1366x640 old apps), replace iPhone 5 and 5s, smooth and future-proof enough with A8 CPU and 2GB RAM.
$199-16GB, $299-32 GB, $399-64GB, on contract.
- iPhone 6 Pro: 6" 2560x1440: 490 ppi, new "maximum size for pocketability", made as a flagship to fend off competition with killer specs and pro features. Applications effortlessly scalable from 1280x720, plus "Pro apps" to take advantage of massive "Desktop class" A8X and 4GB RAM. Screen can split into four 3" 720p squares to show off quad-multitasking from brutal CPU, fandroids commit harakiri.
$499-32GB, $599-64GB, $699-128GB, on contract.
- Lastly, iPhone 6 Air with same guts as iPhone 6 (A8, 2GB RAM), and 4" screen. Lighter due to smaller dimension (mostly marketing), resolution and chassis from iPhone 5C. Available in black or white (please no salmon pink).
$0-8GB, $99-16GB, on contract.