The 5D-III, 6D and the new 70D all use digic5+. With economies of scale digic5+ is becoming cheaper. To me the most obvious approach for the next 1-series body is that it will also have Dual-digic5+. The 1DX data pipeline is good. It could support between 38-40MP at 6fps. This would be an improvement on the D800, both in terms of res and fps. I hope it has improved Dynamic Range at low ISO and that it can offer in-camera HDR-video. If you had to choose one, would you rather have 4K-video or HDR-video?
As a side note, I believe the 7D-II will also be DD5+ based. Let me explain... to be able to compete with Sony and Nikon's hi-speed, 24MP, APS-C cameras, the 7D-II will have to have a 22-24MP sensor and shoot approx 10-12fps. Clearly the 7D-II would cannibalize some 1DX sales. Anyway, if you once again consider economies of scale on the Digic5+, then Canon will probably have some margin to sacrifice to boost 1DX sales prior to 7D-II release. I reckon that if we see a price drop on the 1DX (as some have indicated) then it is a sure sign of the impending release of the long-awaited 7D-II.