Hurray! A positive iPhone 6c rumor! Make it happen, Apple!
My best guess is that the iPhone 6c will:
1. Share the innards of the iPhone 5s (or possibly have underclocked iPhone 6 internals);
2. Have modest additional, tweaked LTE/frequency coverage compared to the 5s;
3. Have a slightly larger battery;
4. Add Apple Pay (NFC);
5. Have the same case design as the iPhone 5c (possibly with a couple color changes);
6. Come in 16GB and 32GB (or 64GB) capacities. Possibly with a controversial 8GB capacity for emerging markets, but I doubt it. If I had to bet: 16 and 64;
7. Maybe (probably not) creep up to a 4.3 inch display if Apple can hold the form factor (i.e. trim the bezel), albeit with the same pixel count (probably) as the 4.0 inch display. Probably not.
Apple could introduce the 6c off-cycle, before the 6s and 6s Plus. There'd be some sense in a tick-tock release schedule for what is now Apple's most important product line. However, I predict the 6c will end up being a 2 year model before the 7c follows, with no (or very minimal -- new colors, for example) refresh in 2016.
Upon introduction of the 6c Apple will immediately discontinue the 4s in remaining markets (really, Apple?), 5c (except possibly in a few markets), and 5s. Apple might also introduce a 6c-based iPod touch at some point, but we'll see. Upon introduction of the 6s and 6s Plus the 6 and 6 Plus will also disappear.
The iPhone 5c is currently priced at $450 (unsubsidized/full retail, 8GB). I think the 16GB iPhone 6c will be priced the same -- or, maybe, due to the strength of the U.S. dollar and ongoing competitive pressures that Apple mostly but not completely ignores, slightly less (e.g. $429).
This is a phone I'd be in the market for -- I'd very seriously consider buying one. I am not in the market for any other iPhone this year.