To those saying why do we need an new entry level full frame when we have the 5D2, I think the point is the 5D2 can't be around forever. It is time to move on.
And why entry level full frame? In the very long term mirrorless will eat away at the low end APS-C models. I think DSLRs will survive, but will be pushed higher end where current mirrorless systems have not gone, excluding the odd ball Leica anyway.
Spec wise, there is little for Canon to cut from a 5D3. It has to move on from the 5D2 otherwise there's no point, but you can't squeeze the 5D3 either.
The easy guesses would be on Digic 5. Limited to up to perhaps 4fps. Sensor derived/recycled from 1D X or 5D3 depending on how they want to play the numbers game. AF... would they go as low as keeping the 5D2 system? Or perhaps put in a 7D like system? Body style, somewhere between the 7D and 5D3. I can't see the 5D2 body kept, but they might not offer all the trimmings of the 5D3.