Ok this is pretty easy:
Take a 60D, upsize the body to support a FF sensor and mirror box, leave pretty much everything else unchanged control wise. Because you're upsizing the body add a slightly larger LCD. Leave it on the hinge, that thing is just *nice* to shoot in the studio with. Clean up any of the minor weather sealing gaps (I don't care it's plastic or mag alloy shell, just don't go Nikon and use a plastic frame).
The sensor should be ideally 28-32MP and support 4:1 (2x2) pixel binning on chip in an optional high sensitivity mode (resulting in 9mp output). If possible I'd be curious to see a WRGB CFA. Failing that a much narrower (spectrum wise) CFA in the red channel would be great.
Burst speed should be around 5 or so FPS. AF system should be a hybrid of the current xxD/7D system. 17 cross type (9 cluster in the center and 8 spread points). Fill with supporting line type expansion points.
Optional battery grip ofcourse, pc sync, hot shoe, ideally integrated wifi support, wireless flash controller (maybe 802.11 flash controller with matching flashes.. please.. Canon.. please!).
Price - ~$4K, product name 3D.
For those wondering what I would do to keep the 1Ds alive with something like the above;
1Ds: 8 FPS, same sensor and goodies as above, 4K video mode, 16 bit ADC for extra bit depth in binned mode. Enormous LCD on the back (come on that's a BIG body). Configurable video codec options.
And what of the 5D Mk iii you ask:
Same sensor as above, without the binning mode. 3.5-4 fps, same AF system as the 3D. No integrated wifi support (for file transfer). Similar design to current 5D physically. Improved sealing (this seems to be a Canon thing now anyway). Same hybrid AF without expansion points.
Oh and all 3 would integrate (somewhere) a speedlite like focus assist beam. Pro or amateur that should be a standard feature. 'My' 5D3 would also have an optional mode that would lock the mirror up, go into live view mode and support EF-S lenses in an on camera crop mode (live view only) to make the upgrade path for the crop users a little bit less painful (seeing as there are now some pretty pricey EF-S lenses out there). All three would support dual cards. The 5D3 would be dual SD (and allow crossfire, interleave file saving, so file 1 on card 1, file 2 on card 2, file 3 on card 1 and so on to overcome SD speeds). 3D and 1Ds would be dual CF.
7D-MkII changes would include a new APS-C sensor featuring a BSI like construction to maximize sensel area. A minor improvement in resolution by an increase to ~22 MP, a much weaker AA filter (which soft anti-moire assist.. note this is not the complete removal of the AA filter, just weaken it). Again a narrower CFA band pass. Improved ADC circuitry to drop read noise. Frame rate increase to 9. Interleaves file save across two card slots. Integrated wifi functionality noted above.
That would leave room for the 70D to regain it's 6.3fps speed. The 70D would inherit the 7D AF system. 70D would share a revised 18mp-2 sensor with the new '600D'. Same technology improvements from the 22mp 7D above, but at 18MP. (then the 80D and 650D can use the 7D-ii sensor while it moves on).
The 600D would inherit the 60D AF system.
The xxxD line would be remade into a mirror-less line with support for EF/EF-S lenes via a converter.
And while I'm at it... Canon would make an aps-c and FF rangefinder like design with very basic lens options (4 primes each equalling 20mm, 35mm, 50mm and 85mm ff equivalents).
But hey.. you asked
