There's a lot of talk about this camera being a 5DIV. In Canon Rumors post from November, it's stated that the new high megapixel camera will come in a new line, above the 5D line, which makes sense to me. The 5D3 is pretty much the ideal all-rounder camera and I can't see Canon messing with what has been a winning formula.
Such a camera would be even more of an "all-rounder" giving the reach of a 7DII, the landscape potential of medium format,and the low-light performance of a 5DIII (or better due to lower fixed pattern noise). They'd just have to find a way to get the frame rates up to the 5DIII's level or better, which should be doable, and provide some options for reducing file size in-camera (cropping, downsampling, lossy compression, etc.).
I agree with lintoni on this: the 5 series is the top end 'general purpose' camera, with the emphasis on 'event' style photography. 23 mp is
high mp, that's the whole point. As private has stated, when shooting hundreds of images, even thousands maybe, massive files are a pain. SRaw etc can reduce file sizes but also slows down processing, so it's far from being an ideal solution. The true resolution that can be obtained from 23 mp on FF when each pixel is true is amazing.
Also 50 mp on FF will not give you the full potential of a larger format, because the larger format always has more magnification, larger capture and greater volume of light for the same framing.
Canon will recognise this. They probably also recognise that there are (just) enough punters out there who desire greater mp on FF than mid twenties to make it worthwhile in producing one, but it will be a separate, more specialist body. Certainly not the 5D IV.