My ideal offseason:
1) Team stays healthy
2) A bullpen. Spend ALL the money we have leftover from arbitration on the bullpen.
I still think there's no way Greinke, Miller and Corbin are ALL that bad again next season, if just even 2 of them turn it around and one of Shipley, Bradley or Ray has a breakout year (the potential is there) then we'll be in decent shape in the rotation.
Our offense (if healthy) should be one of the best in baseball led by Goldy, Pollock, Segura, Lamb, Peralta, Castillo, Tomas, Owings, etc.
3) A bullpen.
If we revamp the pen and our key guys stay healthy then who knows what can happen next year.
I'd expect some improvement from those three (particularly Greinke), but I wouldn't expect anything close to 2015 level success for Miller. His peripherals have always been pretty meh (low strikeout rate, pedestrian walk rate, fairly HR prone). I wouldn't be surprised if Ray has a very solid year (especially if he continues to post elite strikeout numbers) and Bradley continues to improve (his ERA was almost a full run above his FIP/xFIP).
I do think the D-Backs rotation will be better next year.