Having read all of the written and almost all of the introductory videos on the advent of the Fuji X-H1, it's become clear to me that the X-H1 is the first model in an entirely new line of of cameras for Fujifilm, hence the designation "1" after the letter H. Everyone thought the "H" stood for "hybrid", but in Fuji's own promotional content, Fuji uses the term "Hyper", Fujilfilm marketing-speak for "High Performance".
It's important to think about how a camera system is developed from a systems engineering persepctive. The Fuji X/XF line of cameras and lenses has acheived considerable consumer and professional success in a little over 5 years and an represents a very impressive engineering and product development acheivement.
But, Fuji knew that if it was going to truly be able to penetrate the "hard-core" professonal ranks, it would have to develop a completely new series of products, e.g. "long, fast, prime" telephotos; equivalents the to the 300, 400mm f/2.8, etc. pro telephotos used universally by photojournalists and sports photographers, as well as products able to support professional video applications i.e., with their new line of Fujinon MK Cine lenses.
Here's an example to put things into context from a systems engineering perspective: when asked back in early 2017 whether they would make a 200mm f/2.0 or f/2.8 lens, Fujifilm engineers replied that specifically making a 200mm f/2.0 lens would “require an entirely new camera body”. Fujifilm ultimately decided to develop a 200mm f/2 and, as can be seen, this is a big, bad boy:
This meant, as they stated back in early 2017, they would need to develop a considerably more robust and professional stills camera platform, as well as one for suitable for professional video use with their newly launched line of MK Cine lenses (which have been getting rave reviews from videographers).
The result of that is the X-H1. This engineering diagram (diagram 1) from Fuji shows how much stiffer and stronger the X-H1 body is than an X-T2, while minimizing the impact on increased camera size.
I would say the operative terms for the Fuji X-H high performance line is "professional, rugged and robust", which is confirmed by this quote from Fujifilm engineers:
The product planner requested the developers to make the body more robust so that new devices could be installed and the expected camera performance could be realized. In order to make the body more robust, the frame, which is made of magnesium alloys, needed to be strengthened by adding extra thickness. The frame is 125% thicker for X-H1, meaning that the frame has almost doubled in volume (1.25 x 1.25 x 1.25 = 1.95). The strength of the frame is almost twice as strong.
In addition to the increase in size to accomodate the IBIS subassembly, Fujifilm engineers additionally increased strength of the frame and mounting area for mounting long, fast prime telephotos also required additional reinforcement to the diagram shown above. As can be seen in the figure below, Fujifilm added ribs for additional torsional stiffening (see bright areas in diagram 2)
The additional stiffness that is required cannot be overstated, because as can be seen in diagram 3, the new 200 f/2 will exert a lot of leverage on the camera body and lens mount.
Its clear that the X-H1 is not a line extension of the Fujifilm X-T seres, but is the first model in Fuji's new fully professional line of cameras. This is why it resembles the GFX more than the X-T series; why the grip is larger: to accommodate larger, heavier telephotos and Cine lenses. It has a completely new leaf-spring based shutter button and mechanism to improve shutter responsiveness, 100 frames/second EVF and an electronic first curtain shutter to reduce viewfinder blackout times when shooting sports or in the studio. It has IBIS to allow stabilization for video and stills photography when using non-OIS pro lenses like the 16-55 f/2.8, etc. It has a submonitor on the top deck in place of the exposure comp dial, just as on the GFX, because in, professional applications and use scenarios, its much more important to have a display that can be specifically configured for stills or video-specific information. Regarding the comment of “only 15 minutes of recording time"; that's true for the base body, but the X-H1 is really designed to be used with its battery grip, which triples the battery capacity and can be configured in Boost mode to considerably increase camera responsiveness and performance (if you've never used an X-T2 with it's Vertical Power Booster grip, you've not experienced at all what that camera is really capable of).
So, its clear to me now that the X-H1 is not, as originally thought, a "hybrid X-T2”, it is very clearly the Canon 1DX of the Fujifilm mirrorless line.
Fujfilm has now created three fully differentiated and distinct XF-mount lines of cameras in exactly the same manner as Canon:
1) the X-H1 is equivalent to Canon 1D series,
2) the X-T series equivalent to the Canon 5D series
3) the X-T20 equivalent to the Canon 80D series
I don't think that users that like the form factor and features of the X-T series need worry: the line of X-T-series bodies will continue as it has, and X-T3 scheduled to be announced/launched some time in Q3/Q4, 2018.
In summary: not everyone needs a Canon 1DX-level camera, in fact, most people don't. But, many professionals do, and the new X-H1 is Fujifilm's new camera line and peripherals solution for "hard-core, working pros".