Take the M5. Make it a bit larger, of course. Switch the screen to articulating. Put in the best EVF tech Canon can muster. Keep the focusing, it's too good as is. Seal it up. 4k or not, I don't have a dog in that fight but if adding it means an unwieldy battery size then don't add it. Make the grip usable, not a finger pincher like Sony. EF mount, no adapter. Don't make size your first priority, but keep it 6D or a bit smaller. Trigger, high speed sync, no onboard flash. Dual cards. Joystick? Maybe not with thumb touchscreen focusing.