Just because I'm a bit bored today...
Looking ahead to 2020:
Starting pitching. Taillon, Williams, Musgrove, and Archer are practically set in stone - there is a fringe possibility of Archer being moved.
Lyles is presumably a one-and-done with Keller coming up, and I fully expect Keller to be the #5 next season.
Relief pitching. Brault, Crick, and Vazquez are the only currently-healthy bullpen arms under contract for 2020 who look like major leaguers.
Assuming he doesn't fall apart, I would not be surprised if Liriano is brought back.
Kuhl, Santana, Kela, and Burdi should all return healthy to fill in the remaining spots. If that plays out accordingly, the bullpen should be as good as it's been under Hurdle. Still, after this nightmare of a season and that group's history of injuries, Huntington should add a few arms for depth just to be safe.
Infield. Bell, Moran, Newman, and Frazier are locks for the 25-man.
Tucker should leave ST in the 25-man (I wouldn't be surprised if he's quickly a mainstay in the lineup, if not immediately), but you never know with how this organization treats prospects.
As for Kang, I'm unsure of his contract situation. I know he signed a 1-year deal, but I don't know if there are options the Pirates can pick up (according to Baseball Reference, he isn't a FA until 2023, but I'm unsure of the accuracy). That said, I'm going to assume Kang will hit around 20 HRs this year and be brought back. Without knowing if Hayes is ready or what he can do in MLB, I don't see them rolling with him on the 25-man out of the gate, even with Moran there.
Catcher. Diaz is now the starter and should be for the next few seasons.
Stallings I am not comfortable with as the #2; he's a fine #3 but I expect more than a .175 hitter as back-up.
Cervelli needs to retire. His skills are on a rapid decline and he can't go a game without getting hurt. If he doesn't retire, he must be released/DFA'd.
Outfield. Reynolds, Marte, and Polanco seem the likely starters. Marte could be a fringe trade candidate, but given the lack of internal replacement options should they do that, I don't see it happening.
Losing Melky and Dickerson opens up the question: what to do about the lost veteran presence and depth (especially hitting)? It's not all that easy to find veterans who fit in the Pirates' budget who can hit like those two have. Assuming no significant additions are made, we're probably looking at Will Craig and/or Jason Martin filling in. Will they be adequate? We really don't know at this point (though, if things go badly enough, we may by the end of this season).
Overall, I expect the rotation to be very good, the defense to be better (w/Tucker), and for the offense to take a significant step backward with the losses of Melky and Dickerson. It would take another gem falling into their lap to fix that problem. Or re-sgning Dickerson.