For me, one button toggle on and off of precapture is my main desire, however, faster shutterspeeds in Av (< in Av the shutterspeeds for some odd reason max out at 1/8000th, which is not enough to preserve highlights in a white dress or shirt with f/1.2, in bright sunlight) and a proper (read; not resettable, other than in official Canon repair-shops) pincode security that need to be punched every 24 hour, would both be at least equally important....
Yea, faster shutterspeeds can be found in M mode, but with AutoISO enabled, it is for one thing nearly impossible to keep the ISO near 100/below 200, and for another, it is VERY easy to accidentally end up at ISO 4000 in bright sunshine and f/1.2, if there suddently is an image to be captured in the shade, been there, done thát, latest this weekend....;- /
-Possibly the problem is that the R5.2 change shutterspeeds in full stops above 1/8000th, but on the R1, that doesn't happen until 1/16.000th, so at least the R1 could have up to 1/16.000th available in Av, but, I can't see that the shutterspeed changing in full stops should be much of a problem, if AutoISO is enabled...???
And I HAVE set up the C1-3 with Av, M and Tv, and with precapture on, but I got other things that I would like to use the C1-3 for, so being able to assign a button to toggle Precapture on and off, would be perfect....
And as I wrote here earlier, it is really stupid that the +/-and WB buttons on top of the R1 aren't programmable, very few people use them, and they are placed perfectly, in terms of easy access....