I'm guessing that the next iPhone models will go as follows:
Fall 2015: iPhone 6s and iPhone 6 Plus s (could be restyled as 6 + s); features include a sapphire display, better Touch ID, and maybe a higher screen resolution on both models, but that feature might be left for the iPhone 7, and maybe better fit and finish and more color choices like the between the iPhone 5 and 5s.
Fall 2016: iPhone 7 and 7 Plus: completely new design, new features, better Retina HD display (possibly 4K?) on iPhone 7 Plus and Retina HD display at 1920 by 1080p for 4.7" iPhone 7.
Again, these are just my guesses, as all we can do now for future models, and has been the case for all these years, is guesses!