What a terrible situation to start this fantasy.
I am currently a very happy 6D-owner - 14 months on the meter right now.
I am definitely staying in the Canon-camp, as Canon has provided the growth-path, that I needed, since I started digital in 2004 (Canon G5, then 450D, now 6D), and I am very comfortable with the user-interface as a whole.
Should I have to re-start from scratch today, I would probably choose 6D again, as for me it is the perfect balance between IQ, other capabilites and size (I do not want anything bigger / heavier).
However that would change with timing, as we are now quite much into the 6D lifecycle. I know it is not near-term, but of course I would be happy to move into a 6D II or whatever such a thing would be named. The main improvement, that I would look for just now, is a more capable AF-system for action.
My set of lenses serves me well, too. I Intend to stay in the f/4 family (again my sort of compromise), as it becomes clearer, that there will be a strong range of f/4 quality lenses. Here some of the new tools look promising (16-35mm f/4 to replace my old 16-35 f/2.8 Mk I).
Ideas for some changes:
- 50mm f/1.4 upgrade to ... ? I would probably wait for Canon on that
- 100mm L 2.8 macro (bought it when on crop) - I might consider the longer 180mm
- upgrade on flash - from current 420EX to ...? Do not know just now
And then add something longer than my 70-200 L IS f/4.
- a new 100-400mm II ??

- a 150-600mm perhaps (would be my first non-Canon lens)
So overall: Should this ever happen (hope not), please wait a couple of years for some of the vapor to become solid, and for some sort of 6D upgrade to materialize.