What will 'S' and 'C' represent?
Probably multiple things, and they probably won't tell us exactly just like they've never explicitly stated what 'S' means.
I'll echo some of the opinions that I agree with. The 'C' in the iPhone 5C could stand for a number of things; Color, Cheap and CHINA. Color is obvious. In the US many will eat up the color options as stylish accessories to their lifestyle.
Regarding the iPhone 5S, I believe the S will ultimately represent "Security". Security is a hot topic surrounding the leak of Prism and increasingly important as people use these devices more and more. A fingerprint scanner will make passwords obsolete, or at the very least, an added layer of security. It would also finally enable ubiquitous secure mobile payments and could finally make passbook useful.
This brings me to the rumored gold iPhone. Perhaps the security suite will be called "Fort Knox"? I know, probably not given Samsung already uses the Knox term for security features.
What do you all think?