It can't happen on R5 with its (presumed) 45 MPX sensor. There'd even be some crop for 8K. Now, a 6K-to-4K 20 MPX R6 might be the videocentric model in question or there'd be no reason to release it at all.I would be content with 10bit codecs, no 30 min limit and no crop up to 4k30p...
Which probably will be R6, not R5. For $2,500, AF, 10-bit 4:2:2 H264 internal to the CFexpress cards. Low MP count for a video unit; high MP count for photo; some feature overlap.It would be nice to forget about photography for a model or two. Just make a very specific video-oriented system in a stills body, even one that's modified a bit...
Of course, Sony, Nikon and Panasonic have different pricing structure with A7III/A7RIV, Z6/Z7 and S1/S1R - almost exactly a 2/1 ratio. Panasonic has a pro-video S1H too ... but, as I had said, they have no higher end DSLR to protect.