I don’t really see any point in a slow 24-70.Most already have the 20-60 and if they don’t then it is real cheap second user. The whole point of 24-70 is 2.8.
What would a 28-70 f4 add that the 20-60 doesn't more-or-less do? Seems an odd choice to me.
The roadmap suggests a 28-70 vs a 24-70. What the aperture will be is speculation on our part. 2, 2.8, 4.0, 2-2.8, 2.8-4, who knows? If it turns out to be a 4.0, I don't see why that would be an odd choice. For the last few releases, Panasonic has focused on 24mpx sensor cameras, S5II, S5x, S9. Lenses like the 20-60 and 28-200 are great, work well on a 24 mpx sensor, and work OK on a higher res sensor, but start to show their limits. We are all reasonably confident that a high res camera is coming soon. Most likely something similar to the Leica SL3. The S1 series, and SPro lenses are really good, high res, excellent optics, but hardly good sellers. The main criticism is that they are large and heavy. The same criticism with the Leica SL2 and apo lenses. Leica scaled the SL3 (a bit) and rebadged a number of lenses to help address that (somewhat).
It isn't unreasonable to think that Panasonic is thinking the same for the upcoming S1 replacement. If they are planning on relaunching a high res sensor camera, who will it be marketed to? Pros and amateurs who want max resolution image files, whether they need it or not is another question. However, these same people want the best lenses in front of their sensor, but are also (less so) concerned about size/weight.
There are 2 reasons for fast lenses. 1) not enough light and 2) shallow depth of field. New sensors make (1) less and less of an issue (see the aforementioned 20-60 and 28-200 for example). Someone who
needed a 2.8 zoom several years ago, can probably do the same or even better with modern sensors and an 4.0 zoom, lighter, smaller and less expensive.
That leaves (2) shallow depth of field for artistic reasons. In that area, we see advances in AI to simulate out of focus areas. Photoshop offers this feature and ,while by all accounts it isn't perfect, it is getting better (I don't use photoshop, so I don't have personal experience), even phones are adding this feature. It will not surprise me if we start to see that as an in camera feature.
I think the rational for top quality, fast zooms is going to decrease with time, and size/weight/ cost will be a more important factor. As sensor resolution continues to increase, there will always be a pressure on optical excellence and it is much easier to make an optically excellent 28-70/4 than a 24-70/2.8.
Just look at what has happened to the vanilla 50/1.4 in the age of the mirrorless camera. To get the best performance from such a lens on a high res sensor, they are now comically large, heavy and expensive, despite the promise that mirrorless = smaller lenses.