Don Haines said:
I think it will be full EF, but with the ability for lenses to intrude into the body like EF lenses..... oh yes, and you can use EF lenses on it.....
I would guess you are right and would be surprised if they did anything else.
Some years ago Voigtlander made a tiny 15mm DSLR lens that had a rear element protruding - see here:
http://blog.16-9.net/1236-2
This allows a best of both worlds approach. The small and high quality wide angle lenses of the Sony FE mount (like the Loxia 21 that currently DSLRs cannot match in terms of size+quality combination) combined with the superior ergonomics of the Canon EF mount for lenses like a 70-200/2.8 and 400/2.8 etc.
Any other approach making the mount thinner will mean the next 1DX model will likely be inferior in terms of ergonomics - mounting a big white on the Sony FE mount simply is not as balanced as the Canon EF mount - and I can't see Canon wanting to take a step back in the sports and wildlife arenas.
I think future EF mount Canons might resemble more the Sony A99II in terms of size and features but completely mirrorless. Smaller than the current full frame DSLRs but better ergonomics with big lenses than the Sony A9/A7.
Of course Canon might do something completely different. There's no law of nature that says we must always stick to full-frame. In the 1990s photography fragmented and maybe in the future medium format will come back for wedding and landscapes and Canon could introduce a thinner mount like Hasselblad and Fuji for medium format photographers but sports and wildlife shooters will be unlikely to want something physically smaller while at the same time unlikely to want medium format. A 1DX with a big white is pretty much perfect in terms of ergonomics.
Another aspect is simply economics. Over the past few years Canon has spent a fortune on R&D then building lenses like the 35L II, 100-400L II, 70-200 L III, 16-35L III etc - I struggle to believe they would do that then throw the mount away.
I am more curious as to what Nikon might do, I can imagine them trying to roll the dice more, trying to emulate what Canon did in the 1980s with a new mount to reinvigorate their approach as they have less to lose.