I've had Fuji, have Sony (A7r) and Micro 4/3, and am about to re-add Fuji to my bag. The major reason is that the Fuji files are MUCH better than even the most modern Micro 4/3 sensors can manage, and the system is far more compact than Sony's, unless you are using a small prime on the Sony. I find that Fuji holds up well to 36" on the long side with low ISO images (that is on the edge of what it can do), Sony goes well beyond that - I own a 24" printer, but not a 44", so I haven't found the limit, although some cropped panos I've printed in the range of 24x50" look really good. I'm very careful with Micro 4/3 above 12x18, although it WILL print 16x24 nicely on certain types of images.
The reason I'm adding Fuji back into my kit is the weight of the Sony stuff on a hike. I was hoping the Sony 24-240 travel lens would be a nice everything in one solution, but I discovered it's a nearly 2 lb lens (which means it's going to want the grip on the A7r to hold it steady). A7r, grip and 1.7 lb lens is right around 3.5 lbs, more than I want on my neck on a long hike. X-T1 and 18-135 is just under 2 lbs (the lens is light enough not to want a grip), and probably better weather sealed.
Right now, I'm on THREE mirrorless systems (not ideal, but all have strengths)
Sony for absolute maximum image quality
Fuji for very good quality on longer walks - I'm planning to hike the Appalachian Trail as a photographer!
Micro 4/3 for video and very long lenses (the Panasonic 100-300 is a unique lens, in that it has 600 mm of reach in a reasonably sized package).
I'd like to be down to two systems, or even, ideally one. At least for me, here's what it would take:
1.) Sony releases a serious video camera (WITHOUT NEEDING AN EXTERNAL RECORDER) - 4k and slomo in camera, like the GH4 can do. If I had this, I'd ditch Micro 4/3 and use Sony for both video and ultimate quality, Fuji for long hikes (pick up the upcoming 140-400 and use Fuji for really long lens stuff, too).
OR
2.)Fuji releases something in the 24-28 MP range, with the quality per pixel of their 16 MP X-Trans sensor. That would take the place of the 36 MP Sony (and I think the Fuji lenses are up to it - although I'd buy some lenses beyond the 18-135 hiking lens for best quality), and I'd keep Micro 4/3 for video and perhaps really long lens work.
Less likely options:
3.)Fuji releases a camera with GH4 level video. This would let me go Fuji/Sony, or, if the still quality was also exceptional, Fuji only.
4.)Sony releases a truly portable, high-quality zoom lens that extends beyond 70 mm(I'd like to see something like a 28-135 or a 24-105). Variable aperture is OK, but keep it around a pound, so it doesn't need the grip. I think this is unlikely, because I dug into that question very deeply before ordering the X-T1, and saw no signs of such a beast.
5.)A new Micro 4/3 body ends up with class-leading still image quality (I think this is unlikely, both because physics makes it harder with the small sensor and because they seem to be putting more effort into video and body features). They’d also need to have the right lens – neither the Panasonic 12-35 nor the Olympus 12-40 is quite long enough at the tele end for a hiking lens for me (the Oly 14-55 IS, but that’s not as good a lens as most Fuji lenses, for example).
