I'll bite.
I'm guessing rumors in 2019 that will be incorrect, with more correct but opaque rumors in 2020 coming to fruition with a real camera release in 2021, with the following:
New Stuff:
- 40MP
- Disappointingly, 8 FPS (See what they'll do here: increase IQ with MP and keep an inadequate FPS for sports/wildlife so as to protect the 1 series)
- Flippy tilty screen, as the pros will have finally capitulated their weird bias against such screens
- Decent 4k with reasonable codecs
- An improved 1.25 stops of something. This might be low light/high ISO performance, or ISO 100 dynamic range or the like.
- Mode allowing many of the mirrorless features already-then-existing on Canon's FF mirrorless product, such as zebras, focus peaking, high frame rate FPS in liveview, but without AF.
Unchanged:
- Same size and weight, roughly, with pretty much the same interface.
- Price, although adapted to then current exchange rates
X Factors:
- Some new feature that will be interesting, akin to dual pixel AF (huge success) or dual pixel RAW (huge "meh"). This might be a backside illuminated sensor; DP AF information used to inform phase detect in a hybrid focus mode; improved DP Raw feature that uses non-paired pixels to increase the parallax effect, thus making DP RAW actually sometimes useful; etc.
- Some software-based feature that works well with Canon lenses and not third party lenses. This might be auto-MFA or the like.
Market Conditions:
- 30-40 MP will be considered minimum needed resolution for a high quality IQ camera, which will pressure the flagship models to move up in MP faster than the rest of the product lineups. That will, in turn, put pressure on companies to differentiate these models, which will mean a high temptation to nerf the new 5D. Likeliest place to do that is FPS.
- Mirrorless phenomenon will be finally considered a non-relevant issue, as shutter mechanisms will be seen more as a plumbing issue about which real people don't care. In the 5 series, this will be interesting, as it's the most likely camera to be cannabalized by the FF mirrorless offering to come. So there will be pressure to differentiate in a positive way.