Past experience with Canon suggests, no such thing as a 6D, or 4D or any even number for a single digit camera. Won't say it can't happen, just its out of place in their corporate thinking.
The film line had the EOS-1 line, then the EOS-5 (A2/A2e), then the EOS-7 (Elan), then the EOS-3. That has been their pecking order as far as numbers, and sequence go.
As you recall, the "1" was top dog, even without having all the latest greatest features in every feature category.
The "5" line brought eye control and fast (for its time) 5fps shooting. The "7" line had superior focus, and the EOS-3 had all the bells and whistles, but wasn't quite pro-level in build.
I expect the same to be true of the EOS 3D. Higher pixel count... sure. I'd expect something more though. Maybe something like integrated GPS and/or integrated WiFi, and/or integrated wireless flash triggering. Canon's way as been to include some "nifty" stuff on the number 3 in the lineup. I would be surprised if they disappoint this time around.
As the EOS-5DmkII sells off, and along with competition, probably more price drops - OR - instead of a 6D, you'll see a 5Ds or 5Di or something like that in the name. Its Canon's way. So, if there's an "entry level" full frame, expect it to have a 5 in its name, with some other suffix after it.
I see an EOS-3 selling in the sub $4500 league to start, getting sub $4000 rather quickly thereafter.