I bought the Df as a second body (next to D800) and it became first, essentially because of the relative ease of using MF lenses. Or, reversing causality links, the Df made me like MF lenses (I already had several before, but acquired many more since then).
Manual focussing remains tricky using the Df (not that much better than using the D800), but as I am only an amateur, I can live with a few misses. Pros cannot.
To me, the successor will be any camera that will further ease the use of MF lenses. It may well be Nikon's future mirrorless. Unlike a Sony A-something, I'd expect an adapter tailored to AI(S) lenses, so these could be used in normal or stopped-down metering mode. Normal is useful in low light; stopped-down is useful for AF preview and for getting rid of possible focus shift. With sensors ever improving, only stopped-down could become fully acceptable (this is how my old Canon FD lenses work on Olympus m43).
Alternative could be some sort of D850, if indeed the viewfinder as good as advertised in another thread. Or any DSLR with MF assistance that would be better than the AF confirmation dot, and of course head-up style (I hate to look away at the focus confirmation dot, especially with the indispensible DK-17M that will move it further from the axis)