Here are my picks:
C Gustavo Molina
From what we've heard, the Nats are going to call-up a 3rd catcher. Problem is-the three catchers who could possibly be candidates are all pretty awful. Luke Montz, the only candidate on the 40-man roster, hasn't had a multi-hit game since June 20 in AA, without missing any large stretches of time. Jhonatan Solano showed a bit of promise with a .280 batting average in 95 AA plate appearances, but has struggled imensely in AAA, with a .196/.236/.262 triple slash there in 184 plate appearances. This leaves Gustavo Molina, who should never appear on any ML roster, let alone a potential third in the Nats. He hasn't exactly torn up AAA this year, hitting .208/.235/.313. And no, he's not related to Bengie, Jose or Yadier. But given his tiny advantage in terms of ML experience (and the fact that he's regarded as a decent defender by Brian from NFA), Molina gets my vote.
SS Ian Desmond
The past four years or so have been fairly tumultuous for the Nats' (well, actually the Expos') 3rd rounder from 2004. Ever since being compared to Derek Jeter by both former Nats GM Jim Bowden and assistant to the GM Jose Cardenal in Spring Training 2005, the performance hasn't matched the hype behind the young SS from Sarasota, FL. That is, until this year, where Desmond has hit a combined .326/.398/.484 between AA and AAA. With an arm good enough to be named the best of both the International League (AAA) and Eastern League (AA) by Baseball America, the tools are coming together. If he can cut down on his errors and hit even 75% as well as he has this year, he might never go back down to the minors.
OF Justin Maxwell
It's more of a necessity than a deserved call-up, but it's going to happen nonetheless, according to Nationals Journal. It's not that Maxwell is bad-he's already an excellent fielder and has almost identical career minor league numbers to Mike Cameron (Maxwell's .257/.351/.442 to Cameron's .258/.343/.424). He's simply not ready for the bigs, having been overpowered in AAA this year. The 30% strikeout rates need to stop for him to be considered for a promotion, but a lack of CF depth in the system will force the Nats to recall him tomorrow. Norris Hopper would have been a better option in my opinion, but Maxwell is a) the future and b) on the 40-man roster.
LHSP Ross Detwiler, RHSP Shairon Martis and RHSP Collin Balester
Detwiler, Martis and Balester go together because they are almost surely locks to be called up. Detwiler has pitched 22 less innings in 2009 than in 2008, with Martis pitching 3 more than 2008 and Balester 30 less than 2008, so all three have a fair amount of innings left to throw. (Side note-I suggest checking out NFA's September call-up post for the innings pitched breakdowns of all of the candidates.) Brian from NFA noted in the comments of the above link that Balester may be shut down due to poor performance rather than IP counts, which could open a door for Marco Estrada.
RHP Zach Segovia
Segovia has been solid in AA/AAA this year, with a combined 3.34 ERA in 67 and 1/3 innings and a much higher K/9 rate than at any other point in his minor league career (8.2 K/9 overall this year as opposed to a 5.8 K/9 career average and a previous career high over a full season of 6.7. Most importantly, he can eat innings, as many of the current Nats young starters will likely be shut down early. Out of all of the pitchers after Detwiler/Martis, I'd say Segovia is the closest to a lock due to his stamina and experience at higher levels.
RHP Clint Everts
Everts is pretty much the anti-Wilkie. As the 5th overall draft pick in a 2002 draft that saw Royals ace Zack Greinke go 6th overall, Brewers slugger Prince Fielder 7th and Phillies ace Cole Hamels 17th, it would be ideal to squeeze out at least a little bit of production. While Everts' days as a starting pitcher are long gone, he looks to be fairly good out of the bullpen, with strikeout rates trending up and walk rates trending down since being converted to a reliever during the 2007 season. While the Nats don't necessarily have to "save face" by promoting him, it would be nice to see them get at least a smidge of performance from that stud-laden draft.
RHP Josh Wilkie
A Disney story in the making, the undrafted Wilkie has been ridiculously good in his 4-year minor league career. With a career 2.91 ERA and consistently low FIP's (click the link to learn more on Fielding Independent Pitching), he has both the ideal actual performance and expected performance to earn him a spot on the Nats.
I'm admittedly not very high on LHRP Yunior Novoa, RHSP Marco Estrada or LHRP Jack Spradlin, all of whom have put up decent 2009 numbers. If any of the 3 get a call-up, it will be Estrada, as he is already on the 40-man roster.
Other dark horses include LHP Aaron Thompson (needs to be added to the 40-man roster to avoid the Rule 5 Draft this offseason but probably too high in innings), LHP Horacio Ramirez (unlikely due to poor performance with Syracuse), 1B Daryle Ward (unlikely due to poor performance with Syracuse), OF Norris Hopper (possible, but unlikely due to the calling up of Maxwell), IF Seth Bynum (unlikely due to poor AAA performance in 2009, low BB rates and high K rates), IF/OF Kory Casto (unlikely due to lack of sustained power since 2007), 1B Brad Eldred (possibility, but due to the fact that he's an Adam Dunn all power-no glove clone at 1B, I doubt it).
Note-Molina, Segovia, Everts and Wilkie need to be added to the 40-man roster. The Nats are currently standing at 39/40 roster spots, but have up to 5 players who can be placed on the 60-day DL, as they are likely out for the season: RHP Ryan Mattheus, LHP Scott Olsen, C Jesus Flores, OF Austin Kearns and OF Nyjer Morgan. Off the top of my head, Flores has missed enough time retroactively that, if healthy enough, he could be placed on the 60-day DL now and be able to be activated at any point still.