Pentax has always intrigued me - there are several DSLRs that are 24 MP no AA, including those two Pentaxes. What I don't know of (and would be useful for figuring out how much of a resolution advantage X-Trans has over Bayer) is a 16 MP DSLR with no AA. Once X-Pro 2 resolution tests show up, X-Pro 2 vs. D7200 or K3II will be a relatively fair test.
If early reports of X-Pro 2 image quality are accurate, its major competitors are perhaps 24 MP
full frame cameras, rather than other APS-C models. Of course, it's priced like a full-frame camera, so it needs to compete with them. I haven't seen a direct comparison to the K3II or D7200 anywhere, but I have seen favorable comparisons to the D750, which is a (small, but noticeable)step above the APS-C DSLRs.
Also in common with full-frame cameras, the X-Pro 2 has a dedicated lens line for its frame size. Both the Pentax, and especially Nikon APS-C offerings rely heavily on full-frame lenses, which are often odd focal lengths on APS-C, and generally larger and heavier than APS-C lenses.
The competitors are (in something like this order):
Sony A7II - also mirrorless, similar price, same resolution, slightly heavier, full-frame, worse lens selection.
Leica M240 - rangefinder, much more expensive, slightly heavier, same resolution, full-frame, different lens selection (many lenses going back 50 years, no zooms, macros or long lenses)
Nikon D750 - DSLR, similar price, significantly heavier, same resolution, full-frame, heavier lenses, but wider choice
Canon 5DmkIII - DSLR, somewhat more expensive, significantly heavier, lower resolution, full-frame, heavier lenses but wider choice
Nikon D7200 - DSLR, much less expensive, somewhat heavier, same resolution, APS-C, less dedicated lenses, but uses Nikon full-frame lenses
Pentax K3II - DSLR, much less expensive, somewhat heavier, same resolution, APS-C, less dedicated lenses, but uses Pentax full-frame lenses (mostly older)
I've left two obvious competitors (or so it would seem) off the list... Micro 43 was a deliberate omission - image quality isn't quite there even against X-Trans II, let alone X-Trans III. There are several great bodies, and a few terrific lenses mixed in, but the sensor is a significant minus.
While I believe the full-frame Sonys, and specifically the A7II, are probably Fuji's MOST important competitor, I've left the much cheaper APS-C Sonys off the list for a couple of reasons. APS-C Sony bodies tend to be nice sensors with as little money put into the rest of the body as possible. Even the current A6000 is more or less equivalent to an X-A1 or X-M1 with a high-resolution sensor, sitting well below even an X-E2 in body construction and features, let alone the top-end Fujis. I would expect a replacement for the A6000 to venture perhaps as high as X-E territory, although perhaps with an extremely high resolution sensor, and would be a little surprised to see a $1500 range "A7000" with a significantly improved body (both to avoid competing with the A7 series, and because they don't have the lenses for it).
More importantly, it would take quite a few improvements to Sony's APS-C lens lineup to make it competitive (while Sony lists a significant number of APS-C lenses, they are mostly variants of the same few kit zooms, as well as a few low-end primes released in the early days of the NEX line). There is a modestly regarded 16-70 f4, a generally well-liked 10-18, and one or two decent primes. That's about it other than three separate variants of an OK (but not great) travel zoom, two cheap kit zooms and the aforementioned cheap primes.
If they were really serious about top-end APS-C, they'd need (at a minimum):
A replacement for the expensive and not great 16-70 f4 Zeiss - see this
Review of the 16-70A wide prime or two (other than the cheap pancake lenses)
Some telephoto without "f6.3" anywhere in its description, preferably longer than 200mm.
A macro lens longer than 30mm.
Even with these additions , they would still have the Sony 50 f1.8 (or the better, but bulky 55 FE f1.8)up against Fuji's stellar 56 f1.2. The 24 Zeiss is a fair comparison to the Fuji 23, and the Sony 28 FE s disadvantaged against the Fuji 27 only because the latter is a pancake lens of similar quality. Including FE lenses, Sony has 3 35mm options, but the exceptional Distagon is bulky, and the other two are not up to the standards of the Fujinons.
My opinion is that this is unlikely, because Sony has decided to keep their APS-C system essentially for entry-level users. Curiously, they really have no upgrade path - you go from the $500 (with a kit lens on a good special) A6000 straight up to full frame and the $1800 A7II, which still needs a $1000+ lens.