I could see Fuji going FF, to signify their ambition to mix it with the big 3 companies.
Honestly, I don't see Fuji stepping into the FF world anytime soon, and they have repeatedly said so. Their ambition (and that of all µ4/3 manufacturers) is to convert as many DSLR users as possible to their mirrorless platform, and to this end Fuji proposes a pro-grade compact APS-C system with top-quality optics, build and image output. So, with their APS-C sensor X system they have some unique value propositions for the two main classes of CaNikon DSLR photographers:
- APS-C: much better system quality (except for AF, yet) in a slightly smaller package
- FF users: much smaller and lighter package with similar quality (except for extreme ISOs and AF... yet)
Fuji's advantage over the µ4/3 proposition is that their larger APS-C sensor and optimal in-camera data processing result in very good image quality up to ISO 6400 while µ4/3 are good only up to ISO1600.
With a FF sensor X camera, Fuji would lose most of those value propositions and thus the main reason why customer jump from DSLRs to their mirrorless platform:
- the much larger lenses needed would result in a far less compact system
- CaNicon already offer pro-grade full-frame lenses and camera bodies
- they enter in direct competition with Sony and their A7 series
What unique unique feature would be left? X-trans CFA and maybe retro factor or different ergonomy, really not much to survive in this extremely competitive market.
I think Fuji has already realized that they are better served by sticking to their APS-C X-mount platform, where they can deliver on those unique value propositions as long as CaNicon keep on feeding APS-C photographers with second-rate equipment. In the mean time they can work at converting more and more DSLR customers to mirrorless by releasing more top-class lenses and improving further the performances and specs of cameras to come.