The D3 I once owned suffered from the "wtf happens with the aperture follower"-syndrome. A little service job got it right back to working order.
As for a Df successor my list is: D5 sensor and AF, more or less same body, dial locks on or off (like the X-T2), maaaaybe the front command wheel and on/off switch changes, interchangeable focussing screens (not likely but one hopes), memory card access on the right side. That's pretty much it.