I would love 4K raw from the R5, but wouldn't it mean it'd be cropped? Not sure if the R5 can output a oversampled or pixel binned AND Raw image. This is why it already isn't available in camera. Unless the Atomos can do some of the processing? I don't know how that would work.
I doubt overheating will be an issue in a practical sense. There was a time when having the LCD screen on would cause overheating on the R5 even when outputting to a Ninja, but that's been fixed in a firmware update. 4K120 is the biggest culprit to overheating, so it might be technically possible to overheat with an external recorder, but I don't even know if this feature will be available. Currently the announcement says 4K120 is available with a couple Z-Cam models, and Sony cinema cameras utilizing the SDI Kit.