3D - 30-40 megapixels, 7D AF, 3-4 FPS, 100% viewfinder, pro build/sealing. - $3500-4000
5D mk3 - 18 megapixels, 7D AF, 5-6 FPS, less than 100% viewfinder, current build - $2000-2500
FPS and AF seem the most obvious ways to differentiate a high megapixel body from the 1DX to me.
You're right, the main differentiators will likely be fps and AF. The thing is, the 7D AF is the best Canon AF outside of the 1-series. So, for another FF camera to have the 7D AF would not be a real differentiator. Now, maybe it would...if you really mean the actual
7D AF, i.e. an APS-C-sized AF sensor - that would have the same top/bottom spread as the AF in the 5DII, and a slightly reduced side-to-side spread.
But, I'm pretty sure that you mean the 7D's AF system upscaled to match the FF sensor, meaning even though it only has 19 points, it would covers the same horizontal spread as the 1D X and would even have a slightly larger vertical spread, i.e. much
better coverage than the 5DII. The 7D's AF is good enough that Canon borrowed many of those features for the 1D X. They're not going to put an AF that good in a FF body outside of the 1-series, because then they'd be differentiating primarily on fps, and that's not enough.
Consider where the 5D/5DII's AF is positioned in the lineup - it's based on the 20D's AF and thus most similar to the AF system currently in the Rebel/xxxD line (9 points, only one cross-type). I'm ignoring the 6 invisible points on the 5D, since those are intended to help with AI Servo tracking, and while maybe they do help, tracking a moving subject is clearly not the 5DII's forte). In overall performance, the 5DII's Af is a step below the xxD AF. I think a sub-1-series FF will keep the same relative position. It seems likely that the xxD AF will get a bump - it's been used in three bodies (40D through 60D). So, Canon enhances the xxD AF slightly (say, 15 points and better tracking), gives 9 cross type points to the xxxD/Rebel line but none of the sophisticated tracking (and that addresses how they can release a new xxxD in early 2012 with the same 18 MP APS-C sensor), and the next FF gets the FF-sized version of that, with no broader spread of points than is currently in the 5DII.