I think it might be a misstep for Panasonic to create the three separate lines in this manner. To me it screams being a reactionary move instead of well-thought marketing strategy. Say this new camera will be a heavily video-centric model, the G9 is the heavily photo-centric one, and the GH5 is the hybrid. The problem is that throughout its marketing cycle, the GH5 was presented as a video camera first and foremost, with some talk about the stills features. Remember, the first tease of info about the camera was solely video based: 10 bit 4:2:, 4K60p, and 1080p180. Even the in-person interviews with Matt Frazier and Sean were so heavily leaning on video that stills were barely even brought up. If Panasonic had this 3 camera plan all along, the reps surely would have emphasized the *hybrid* designation more (even if they weren't going to announce other cameras for some months).
It seems disrespectful to consumers of the GH5 for Panasonic to release a substantial updated model so shortly after release. Especially, if this was on their roadmap all along. This would be unlike Panasonic. Therefore I think a likely scenario is an updated model with a few new bells and whistles, but with the bulk of features coming from software. Panasonic will offer these software upgrades to GH5 owners at a cost ~$500.
Less megapixels is certainly not the only way to increase ISO performance. The GH5 is substantially better than the GH1 in low light and it has far more megapixels. Processing power, heat dissipation, and other factors also have a role in ISO performance. The fact that Luke Neumann is posting high resolution anamorphic frame grabs points to the same or a variant of the GH5 sensor.