Here's the most logical (IMHO) lineup for Fall 2015:
iPhone 5C (upgraded): $0 on contract, 8 and/or 16 GB storage (maybe finally drop 8GB), internals from iPhone 5S (A7, M7, same camera with pill shaped True Tone flash), TouchID, same color options as current 5C, remote possibility of NFC for full Apple Pay support (expand the user base)
iPhone 6: starting at $99 on contract, same model as 2014 (A8, M8), 16 and 32 GB storage, Space Gray, Silver, Gold
iPhone 6S: starting at $199 on contract, upgraded internals (A9, M9, faster storage), 2GB of RAM, 32, 64, and 128GB options (maybe still 16 at low end instead of 32?), minor camera revisions (doubt drastic changes, like dual lense or optical image stab.), upgraded TouchID, major addition of Force Touch and Taptic Engine, addition of rose gold color to gray, silver, and gold.
iPhone 6S plus: same internals and upgrades as 6S, Force Touch and Taptic, Rose Gold color available.
2016 may be the year the 4" model disappears and iPhone 6 internals (A8, M8) get crammed into a plastic housing, with the rest of the line up being iPhone 6S, iPhone 7 and 7 plus (A10, M10, new design), all with ForceTouch and Taptic Engine.