We're told that the EOS 6D Mark II will not come until after the EOS 5D Mark IV begins shipping, which will be into the first quarter of 2016. This would follow the same timeline as the EOS 5D Mark III & EOS 6D announcement.
The EOS 6D Mark II will have a slightly smaller and lighter body. It's suggested we may actually see it use a different battery than the LP-E6/E6n to help with weight reduction. There won't be any radical changes to the body such as an EVF or articulating LCD. Expect the megapixel count to rise, but not above 24mp. It's likely the EOS 6D Mark II will have its own sensor just like the original camera.
We've received very little about this camera over the last few months, so I suspect the timeline for an announcement is accurate.