The main advantage I see with the 5D3 over the 5D2 is the auto ISO in manual mode and this has really made it more of a sports camera now than just a wedding/portrait camera. You can now shoot sports with spot metering with a fixed aperture and minimum shutter speed. This would be unheard of with a 5D2.
... of course Canon is just catching up with the crop cameras 60d, 7d etc there - I'm sure all upcoming Canon dslrs will have it.
The AF sensor issue you talk about is true, don't get me wrong, but there are far less total AF points on the 60D and 5D2 that those cameras don't really factor into serious sports shooters' thoughts.
Absolutely, the 60d just sucks at servo af, zero customization and little number of af points - but for amateur sports shooting many people will get a 7d due to the crop factor (= cheaper lenses), and pros probably will use the 1dx (faster, more precise af than 5d3) or 1d4 (1.3x crop). That's why I don't have the 5d3 in mind as a "sports" camera at all, but of course it's very usable for this purpose.
My point is this. If we would see a 6D or something come out, it is very difficult to predict what type of camera it will be.
The 6d will be placed as a Nikon d600 competition and as a more modern 5d2 replacement. And there will be a 6d, because it is a real novelty to keep the predecessor 5d2 alive alongside the successor. And Canon won't get away with their cheapest ff body priced as the 5d3 is now, the market pressure from below is just too high: If you can get very cheap bodies with the current 18mp aps-c sensor, only a few bird or budget sports shooters would buy a updated premium aps-c body.
Given that, imho the 6d won't be a surprise: 5d3 sensor, less sturdy body, slower fps, single card, less af points & cut servo af firmware, maybe swivel screen and no joystick but a multicontroller like on the 60d. And a lot of tiny hidden annoyances placed there by Canon marketing to make you with for a 5d3 :-o ... but anyway for under $2000, it'll sell just fine.
