OK, a prediction I haven't seen elsewhere:
iPhone 6/6s has had the same "@2x Retina" screen quality that first came out in the iPhone 4 with 326 pixels per inch.
The 6 Plus and 6s Plus upped that with a "@3x Retina" screen with 401 pixels per inch.
Not sure if the bigger 7 Plus will go up to a 4x Retina and 535 pixels per inch, but I'm predicting the 7 will bump up its screen to a "@3x Retina" with 489 pixels per inch.* Better screen, faster, better camera are always good enough reasons for people to upgrade.
(And that's on top of what others have guessed with be a Higher Color Range screen.)
* Exactly how it does the 6 Plus screen does scaling is a little different, that's why its 3x and iPhone 6's potential 3x aren't the same.