Tim already took a shot at who the Pirates might promote when rosters expand. Here's mine. It could be a bigger group than usual.
P Charlie Morton: Has a rehab assignment in Altoona tomorrow. He'll obviously be back whenever the Pirates deem him healthy.
RP Stolmy Pimentel: He's currently rehabbing his very conveniently timed ankle injury, and he's out of options. There's no reason he can't be on the roster as long as he's pitching in garbage time, and it's much easier to make sure he pitches garbage time if there are more than seven relievers on the roster.
RP Casey Sadler: He's on the 40-man roster, he's already pitched for the Pirates this season, and he's pitched pretty well for Indianapolis.
RP Vin Mazzaro: He's not on the 40-man, but the Pirates can use a capable reliever, and there's no shortage of players the Pirates can designate for assignment to clear space for him.
RP Bobby LaFromboise: I'm less certain about LaFromboise (who the Pirates recently claimed, in case you missed it) than the others here, but since he's left-handed and is on the 40-man roster, there isn't much harm in the Pirates adding him as an extra tactical option. I do wonder if Rafael Perez or Andy Oliver might be better pitchers, but then I also didn't really understand why the Pirates claimed LaFromboise in the first place.
C Tony Sanchez: A third catcher? Why not? I wouldn't expect Sanchez to play much, but there's no reason the Pirates can't carry him in order to free Chris Stewart and his .365 OBP to pinch-hit. (That's a joke, kind of.)
INF Michael Martinez: The Pirates have already let him occupy a bench spot for exasperatingly long stretches this year, so I'm not sure why he wouldn't be up in September, despite his total lack of offensive ability.
3B Brent Morel: It's one thing to hit badly, as Martinez does. It's something else entirely to hit badly and be confined to one position. Still, Morel will probably return to the big leagues, especially since Jayson Nix's DFA makes Morel a less likely DFA candidate himself.
OF Gregory Polanco: An easy call.
OF Andrew Lambo: The Pirates may not like Lambo as much as we do, but since he's on the Bucs' 40-man roster and has demonstrated that he's unlikely to be overmatched by big-league pitching, there's no reason for the Pirates not to carry him, just as they did last September. Just don't expect him to play much.
OF Jaff Decker: With two better lefty outfielders in Polanco and Lambo also likely to be among the Pirates' callups, there's no clear way for Decker to get plate appearances, but I'd still expect the Pirates to carry him.
Possible, but less certain
RP Ernesto Frieri: I'd like to see the Pirates bring Frieri back on a minor-league deal next year, but now that he isn't on the 40-man roster, I can see them passing on him here unless they have reason to think a couple weeks at Indianpolis have helped him figure things out. It's worth noting, though, that he's still signed to a guaranteed contract, so I could see the Pirates shrugging and calling him up despite how badly he pitched this summer.
RP Andy Oliver: Has gotten a ton of strikeouts and has a sexy ERA this year, but he's not on the 40-man and, as with Frieri, would probably have some gas-can tendencies in the majors.
RP John Holdzkom: Differs from the other players in this section in that he's an actual prospect, a big-bodied flamethrower who isn't that old and has pitched very well for Indianapolis. I doubt he's ready for high-leverage big-league innings, but it would be exciting to see him.
P Rafael Perez, P Josh Wall, P A.J. Morris: In other years it might be easier to see the Pirates calling up guys like these as rewards for jobs well done in the minors, but with so many other candidates, I'd bet against it this season.
1B Chris McGuiness: He's on the 40-man, but the Pirates have shown no interest in him since they traded for him, and they'll have eight billion lefty corner players to choose from. He's a potential DFA candidate.
INF Jayson Nix: He could be a possibility if he gets through waivers and winds up back in the Pirates' system, although, with Martinez and Morel occupying 40-man spots, Clint Barmes healthy, and at least Mazzaro needing to be added to the 40-man, the Pirates might just pass rather than adding Nix again.
Maybe next year
OF Mel Rojas: He looks like a prospect to me, although the Pirates haven't always played him consistently at Indianapolis and have no real need for him at the moment. I'd be surprised if he were promoted.
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UPDATE: There have been questions about Nick Kingham and Chase d'Arnaud in the comments. Kingham has pitched sparingly at Triple-A, has pitched a bunch of innings this year and is headed for the Arizona Fall League, so I highly doubt he'll receive consideration. d'Arnaud, meanwhile, isn't on the 40-man roster, and it sounds like Neal Huntington isn't much of a fan of his.