The current 6D is essentially a 5D Mark II 2nd Gen.
If Canon stays on this path - the 6D2 will be a 5D3.
But will it be better?
I think high ISO noise will improve. Not sure if they'll officially claim it is a stop better - but IQ will get better.
Megapixel count might go up no more than a few.
I can see the FPS going up to 5, or maybe 6 at the most.
I can see the AF going up to 19 points. Not sure about 61. That would just put it too close to the 5D series.
Another item that is hard to predict is dual or single card slot. Depends how low Canon wants to be about crippling a camera to prevent pro use.
Buffer could increase, after all, memory only gets larger for the same or less money.
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I've speculated in another thread that while the 6D was equal or better than the 5D2, there are reasons why the 6D2 may not follow suit and be equal or better than the 5D3. In short, the 5D3 was a huge leap in performance, particularly the AF system.
Like all Canonistas around here...there's always a lot of hope and excitement, and the speculation often goes way above what Canon actually produces. The 5DS is really nothing more than huge megapixel count. The rest is just a 5D3, and with weaker ISO. The 7D2...this was a let down for many who hyped the specs. On sensor, ok nothing huge. But on AF and FPS - Canon did great.
Seems that Canon is not going to lay out any single camera that has multiple features that are significantly improved.
5DS = Big MP Sensor
7D2 = Speed and AF
5DX/5DC = Video? Low light?
6D2 = Low end FF general purpose camera (a 5D3 with some things better, other things worse).
1DX2 = Everything except for big MP.