Canon have basically dug themselves into a hole with the 5DIII specs and price:
- If they are planning a more expensive model than the 5DIII with more MPs (say a 5DX), then this model will not be competitive with the D800 on price.
- And if they are planning a lower-priced FF camera than the 5DIII with less MPs, then this model will not be competitive with the (rumored) D600 on megapixels.
So, it indeed appears that Canon's most sensible option for a lower-priced FF camera would be to put a higher MPs sensor (say 28-30mp) in a body with the 7D AF system.
This will provide enough differentiation from the pro-oriented 5DIII and will be competitive with the rumored D600.
... Except that they won't do it, IMO.
I think Canon will just wait for the D600 announcement and will reduce the 5DIII price.
As I've predicted many times, the 5DIII price will drop to $2999 this Christmas and $2499 the next.
So, there you have it: Canon's "entry level" FF camera will be the 5DIII @ $2499 at the end of next year.
After that, it's anyone's guess.
If the D600 is successful (very likely), Canon might respond ... eventually.
For now, though, the 5DIII will be Canon's only 'affordable' FF camera.