Haha, just mark my words 
The 1DX2 is a given and they can't really release a 5D4 first and out-perform the current 1DX.
I reckon the 5D4 will get a megapixel bump to around 28mp and the new 65-point AF from the 7D2, not sure what else. The biggest question in my mind is whether it'll be the first FF body with a dual-pixel sensor for video AF.
The 1DX2 is a given and they can't really release a 5D4 first and out-perform the current 1DX.
I reckon the 5D4 will get a megapixel bump to around 28mp and the new 65-point AF from the 7D2, not sure what else. The biggest question in my mind is whether it'll be the first FF body with a dual-pixel sensor for video AF.