I would add 802.11ac and 2GB RAM as the likely additions to iPhone 6.
Regarding the screen itself, I seriously doubt Apple would change the pixel density.
Retina iPhone, iPod touch, and iPad mini all share the same pixel density for one reason, they are meant to be held with one hand and thus viewed at similar viewing distance (12 inches or less).
My guess is that Apple will again use the home screen as the guide for increasing resolution, while maintaining same 326 pixels per inch density.
To put in a layman's term, home screen will go from 4 by 5 icons (plus 4 icons in the dock) in iPhone 5/5s/5c to 5 by 6 icons (plus 5 icons in the dock). This translates to 800 by 1422 resolution and 5-inch screen.