Here are my speculations
The new mount will be called the RF mount
The future might be RF and EF-M Vs today's EF and EF-S
In order to use RF lenses on EF-M cameras you would need a new adapter. I am assuming the RF mount diameter will be 54mm same as the EF mount. (EF-M mount diameter is 44mm)
M.ADAP R --> Regular EF-M to RF adapter
M.ADAP R ND --> EF-M to RF adapter with ND filter (variable most likely)
M.ADAP R PL --> EF-M to RF adapter with CPL filter
Is there any patent from Canon for an electronic variable ND filter or an electronic CPL? if so that would make sense for the 3 variants of the filter. But it does not makes sense to make such filters for the EF-M mount.
Either way if the ND and CPL are electronic filters or not and if that is indeed the RF to EF-M addpter with filters Canon might have some plans for a great EF-M video camera in the future
On the specs of the camera
Most likely the new Full frame MILC is a lower end body i.e. the equivalent of 6D series.
Only one card slot. Hope they use SD UHS ll. I don't think Canon will use cFast or CFExpress in a lower end body but I would not be surprised if they decided to use CF cards. I know this might flare up a lot of criticism but most Canon 7D, 5D and 1D users should have CF cards already. one beauty of CF compared to SD card is CF is better at sustaining read and write speed at least in my practical use.
~28 MP.
7-10 FPS.
Variangle LCD display 50-50 chance.
4K30fps 50-50 chance.
If 4K is true chances of MPEG-4 AVC/H.264 are 50-50
AF in stills have to wait and see how this compares with DSLRs and other MILC competitors. I would not be surprised even if AF does not live up to 6D2 level.
DPAF in 1080p is obvious but 4K is 50-50 chance.
Don't be surprised 4k has the crop like 5D4. If this is a 24, 26 or 28 MP sensor then the crop factor will be less than 5D4
Lens mount might be thick same as EF mount.
RF mount will be same as EF but the RF lenses might protrude in to the body (meaning the mount is not at the rear end instead at about 28-32mm from the rear end in to the body of the RF lenses wherever necessary) to make a short flange distance. I know a lot of folks will hate this size camera but I don't. In fact I would like this body if this is as big as a 80D or a 6D2 because that will mean better ergonomics for me. I know that does suits all. I do not believe in the notion of MILC is for small bodys.
If the RF mount will be indeed like I guessed the overall length with a wide angle lens mounted on the body will be on par with other MILC systems. At the same time will not have an unnecessary empty tube at the end on the normal to tele lenses like other mirrorless lenses which will will the lens size competition

Since the RF mount can take EF lenses there is no need or a RF to EF adapter.
BG-E22 is body grip for this new camera. Sounds obvious.
I hope RF lenses are not Focus By Wire. If it is FBW I will whine about that but I will adapt them anyway
If body is in deed a lower end body then RF 28-70F/2L and RF 50F1.2L does not makes sense but that will make sense as soon as Canon comes up with the next high end body.
That is a very conservative expectation knowing Canon is too conservative but I will be here bashing

Canon if they do not even live up to these specs. That applies only for the camera specs not about the RF mount or adapters.
Disclaimer: All specifications are my wildest speculations and are subject to change without notice
Now these specs are true that will disappoint a lot of people and Canon is doomed will be heard all over the place but Canon will sell them by boat load because it is Canon well... may be not this time around.