I'm thinking the 5S may disappear with the launch of the 6. I agree with the above post that the insides of the 5S with touch ID will become the 6c. Meanwhile the larger two new 6's need some name to differentiate them, maybe iPhone 6 and iPhone 6Air??
Either way I see it as medium, large and Phabulous!
Apple held onto the 4S for emerging markets, so maybe if they hold onto the 5c (instead of the 5S) they can fill that niche.