We're told that the Cinema EOS C500 Mark II is going to be announced some time in mid 2016 and that the camera will likely shoot at a resolution “well above 4K”. Canon “knows it was very late to the 4K market” and does not want to be considered late to the 6K+ world.
The form factor of the camera may change as well, as we've heard constant rumors of Canon doing a shoulder style camera in the Cinema EOS line.
The C500 was announced in August of 2012, which was nearly 8 months after the C300. By most accounts the C500 didn't sell all that well, unlike the C300 which exceeded expectations. It stands to reason that it'll have to be a more advanced product to justify the price and to have any hope of competing with ARRI.