I suspect that we will only see new C-class hardware (as in iPhone 5C), in "refinement" years; odd numbered years, or "S-class" phone years. It makes sense if Apple's goal is to no longer offer 2 different phones with the same form factor as they have in the past. The 5C achieves this by replacing the 5 and being sold along side the 5S. Since next year will supposedly be a new design year, we could expect the 5S to fill the $99 on contract spot, since it wont be sharing the exact same form factor as the new flagship phone (iPhone 6, or whatever they call it), and the 5C will be the free on contract offering. The following year is when we could see a new C-class phone for $99 on contract to be offered along side the 6S (or whatever), and the 5S would occupy the free on contract slot like the 4S does currently. Basically I think the lineup will look like this for the next few years:
Currently:
iphone 5S: $199
iphone 5C: $99
iphone 4S: free
2014:
iphone 6 (new design): $199
iphone 5S: $99
iphone 5C: free
2015 (will look a lot like this year's lineup):
iphone 6S (same design as 6): $199
iphone 6C (new, plastic design): $99
iphone 5S: free
In each case, the 3 phone tiers each have their own hardware design. Which I suspect might be one of the goals of Apple's 5C offering.