What I love about this whole topic is the disconnect with the reality of how Canon deals with its product cycles.
I see so many saying that the 1dx is being replaced or that this will be the successor to the 5d3. What kool-aid are you guys drinking? Maybe in the rebel world we would see such a rapid move to push out a new product, but not with pro level gear, or near pro level (XXD line). Canon hasn't put out a replacement for the 60D yet, which is a line that gets updated every 1-2 years. So to think we'll see even a rumor for the 5d4 in 2013 is a dream idea - more likely we'll see the dev rumors in 2015, with a potential release in 2016. (Id say the idx will follow a similar pattern...
Big MP --- I am pretty positive that a big mp body will be aimed at a niche audience - studio, landscape, architecture - where the 5d3 is aimed at weddings and event shooters and the 1dx is for wedding, events and sports (5d3 can do sports too, but the slower frame rate).
tech is advancing, but, I keep hearing it from my nikon friends --- while the d800 is a great camera, it's niot their body of choice for weddings. Most rely on the d3s, or d700. I know a few who roll with the d3s and the d800 - but the d800 only comes out for the formals portraits (reasoning - because thats where the strength of the big mp sensor currently is (Low ISO shots), the advantage of the big MP's goes away once you get into the receptions.
I think nikon jumped the gun a little bit with big mp's by trying to make such a camera to act as an all around work machine.
Just a shot in the dark here, but my guess is that we'll see 2 big MP bodies in 2014, a 1d style and a 5d style (not sure what they will name them). Both will probably get the 1dx AF system, digic 6, one will be in the 8k range and the other in the 3-4k range. Neither will have fast frame rates, guessing 2-4 frames per second (1d style will probably get the 4 fps, 5d style would get 2 fps). Emphasis would be on IQ not speed. This would enable canon to funnel this tech into the next round (yes, this would be the 1dx2 and the 5d4). it's a sensible plan - get the new sensor out into its niche market, then by 2016 we'll have digic 7 or 8, our PC's will be that much faster, memory card speeds will be faster and their capacity will be greater and their cost will go down, how about some usb 4 maybe? (Imagine transfering over 2000 45 MP images via usb2, lol, you have lost productive workflow time before you can even begin the workflow at that rate!!!!) -- all of that will lead to bigger mp's in the 1dx2 and 5d3 without comprising on the speed or high ISO performance because how many sports shooters (or wedding shooters) will want to deal with less fps, and a smaller buffer??? (flip that coin too -- DR, yeah, we want more DR, but DR goes away as the ISO increases which is where the current 1dx and 5d3 shine).
I'd much rather wait for the other tech to catch up so we can enjoy the benefits we currently have in canon bodies + more MP's, rather than compromising on certain things like one must do with the current nikon offerings...(again, nikon wedding shooters are hunting for d700's because the price gap to the d3s is too steep, the d600 isn't as good as many thought it would be and the d800 isn't what they hoped it would be).
LOL, the grass is always greener - on the canon boards it's all about more DR and MP and canon has fell behind, on the nikon boards it's people wishing they could have great high ISO with reasonable files sizes and in the 2-3k price range...and that ain't there for them...