For the next Fujifilm X-Pro (X-Pro4, 5, or X-Pro6) we will see its real name when it finally comes out.
What would you like to see in this future X-Pro? How many megapixels, the characteristics of its screen(s), the viewfinder modes. And its price of course!
Unless Fuji's engineers are planning more exotic design surprises like the X-Pro3's flip screen and use of titanium, then it seems to me that the basic design pattern to follow is pretty obvious, namely, that of the very successful X100 series. Of course, being an X-Pro, we'd expect it to be a little larger, a little heavier, more robust, and with a more pronounced grip.
Many of the original X-Pro1 users seemed to regard the X-Pro2 as being close to ideal, addressing as it did many of the shortcomings of the original camera while maintaining its key qualities. The X-Pro2 possesses a brick-like solidity, made possible by its unitary design (i.e. no flip screen). If push came to shove, you really do feel like you could use it as a force multiplier in a life threatening situation down some dark alleyway. It wasn't perfect of course, the EVF wasn't great and the spring loaded pull-up ISO dial was awkward to use, but these are not major engineering challenges and both have subsequently been addressed, either in the subsequent Pro3 or, in the case of the ISO dial, in the later X100V. (I suspect there will be a lot of griping if Fuji's engineers don't adopt the X100V's superior, pull-up dial for the next X-Pro!)
That unitary design had another benefit, it made for smoother lines and more comfortable handling. The bottom left edge of the X-Pro3 has a relatively sharp corner where the flip screen hinge juts out slightly from the body. If you only use the camera for 10 minutes here, half an hour there, this probably won't be an issue. But if you use it for many hours, that sharp corner makes its presence felt and can get pretty uncomfortable.
The same goes for the official metal handgrip. The more chamfered, smoother edges of the Pro2's grip made for more comfortable handling. In particular, the right, rear corner of the Pro3's grip has an inexplicably sharp edge which you really notice when held for many hours. Ditto the top left corner of the EVF housing that nestles into your eye socket. The X-Pro2's EVF housing has a less abrupt chamfered edge, whereas the edge of the housing on the Pro3 has a sharper 90 degree angle that, once again, you really notice when using for hours at a time.
These may seem like relatively minor issues, but IMO, with the X-Pro3, they add up to a camera that is ultimately less comfortable to use than the X-Pro2, particularly when used continuously for half a day or more. IIRC, when Kevin Mullins (ex-Fuji Ambassador) was asked by Fuji's engineers what he wanted to see for the X-Pro3, one of his requests was that he wanted the viewfinder to be a place where he could comfortably hang out all day (or words to that effect). I feel the same should be the case with the handling of a camera, so I was a little disappointed that some of the smoother edges of the Pro2 were replaced by more angular edges on the Pro3, for reasons, one suspects, that were largely to do with appearance rather than ergonomics.
Other improvements I'd like to see for the next X-Pro include less stiff dials. The effort required to rotate the rear dial on my X-Pro3 is almost the same as the effort required to activate its click function. So when I rotate the dial to incrementally zoom in to images in playback mode, I often inadvertently activate the click, resulting in the zoom function being cancelled. Not a show-stopper of course, but irritating when it happens repeatedly. Another thing with the Pro3's dials is that they're relatively noisy! The dial's rotary clicks are quite audible, sufficiently so that I've had people in quiet environments look round when I'm using the front dial to adjust my shutter speed. You need very good hearing to know when I'm rotating the dial on my X-Pro2.
Finally, it would be great to see a return of the X-Pro2's OVF magnifier to better support the three "traditional" street photography focal length equivalents, 18mm, 23mm and 35mm. This provided the two magnifications necessary to accommodate the 18mm field of view as well as provide a reasonably expansive 35mm brightline frame. I don't know why the magnifier was removed. Perhaps for reasons of cost and/or engineering complexity, or perhaps Fuji's research showed that people just weren't using it often enough to make it worthwhile maintaining the feature? Dunno. Whatever, there's been no indication from Fuji that it's going be reinstated, but IMHO this would be a shame. It was such a clever, effective feature that helped make the OVF more practical.
Well, this has ended up rather longer than I was intending. I haven't addressed your questions about megapixel count or cost, and I haven't even even discussed the future of the flip screen or the X-Pro3's gratuitous use of a very expensive and difficult to work material, titanium!