Good news - the GH5s already gives you an option to shoot 400 megabit all-I in 10-bit with vlog-l. For about 99% of scenes, that will be about as good as shooting raw, but with smaller file sizes (also, the 150 megabit long gop is also really good).

The real solution is to buy the Blackmagic 4k Pocket camera when it comes out (hopefully) in September. It's a micro 4/3 camera that will record 4k RAW up to 60P internally and externally.

Everything is possible. Unfortunately nobody is hacking it.

@Ralph_B i am keenly awaiting the new BMD 4K PC. only two points: 1) as yet i have not managed to find a single frame of demo footage from the camera, which i find a bit strange, and 2) BMD don't always quite hit their camera delivery deadlines (though since they are using the sony sensor, maybe they won't have so many sensor issues), thus, i am not expecting the camera to be available for delivery before xmas, though i would be very happy for everyone if it were.
@endotoxic the Z Cam E2 is apparently due to start shipping in July. it doesn't appear to offer raw and its focus seems to be on 120fps, which is great for some people, but not the main issue for shooting cinema fiction. i would like to know whether the camera shoots log (cf the terra 4k shoots only log). the idea of an SDK is good. it will costs less than a GH5S, though obviously it will require some kind of viewing screen. it will be very interesting to see how the E2 goes. some E1 users sounded less than enthusiastic about the E1, but that doesn't mean that the E2 won't be a lot better.
@eatstoomuchjam yes, maybe there won't be enough difference to make it worthwhile. in theory, there should be quite a difference - i hope to be able to make the footage available for download so that anyone can test it if they would like to.
@tonalt perhaps it is just that there is little room for codec improvement in the GH5/S given the processing power in the camera.

What do we know about the color gamut of these two forthcoming cameras? Is it strictly REC709? If so, no way to produce HDR videos?

@IronFilm - after a few minutes further searching, this was all i could come up with:

11 seconds and running at 12fps. if you have even one link that that is better than this, i would be most grateful if you would share it! also, if there is a site that has some raw footage to download that would be excellent.

Ditto what @Ralph_B says. Blackmagic 4K Pocket for raw.

raw creates huge files and workflow problems it is true, but for pulling a really good key, i would like to be able to see what the difference is between the various possibilities of raw, 444, 422, 12 bit, 10 bit, 8 bit and various kinds of compression. this will take time, but some reference raw material will be very helpful, even if we end up sticking with the GH5S and AVC all-I 10bit 422. if this combination is good enough (for cinema level keys), i shall be delighted, but the pundits say it will not be. i hope to find out soon whether they are right or not.

IMHO, either GH6 is going to be the next intermediate like a GH3 between real models, or it is going to be a big surprise for everyone. There is some interesting behavior around the camera scene in Tokyo at the moment. I think each company has it's eyes on the prize for the next big round. Will be very interesting to see how close in step they are with each other.

Depending on the external recorder you use, you could record with a lossless codec (cinemadng) or prores (probably not a huge difference from the internal all-i, but I don't do a lot of keying so maybe it's a bigger difference than I expect), but also 4kp60 is only 4:2:0 8-bit internally and you can record it as 10-bit 4:2:2 with an external recorder.

I bet you're excited to hear the news on GH5s getting the external prores raw support... its a suspicion I've had for a while but somehow Panny just acts way late on things to push new hardware. Most GH5s users have already moved on... If braw is so similiar in bitrate i'd imagine itd be possible as well.