6S and 6S Plus. Same physical design. A9 processor and M9 motion coprocessor. Should match or exceed the performance of the A8X in the iPad Air 2, so the A9 could possibly be a 3 or 4 core, and maybe have 2gb of ram. Improved iSight camera. (Maybe OIS will make its way in to the smaller 4.7" model) Maybe a slight battery increase. If Apple can get the supplier lined up, I hope they will use sapphire screens, like they originally planned to in the 6 & 6 Plus.
On the low end, I think they will either make a 6C, which would just be a current 6 with a plastic body... Or a revised 5C, which would essentially be a 5S, with the 5C plastic body.
I think the 4S and current version of the 5C and 5S will be no more. iOS 9 will likely cut off the 4S, but will probably support the 5/5C forward. But they could cut off all pre-64 bit devices, including the 5/5C, and focus on the 5S, 6, 6 Plus, and 6S and 6S Plus.