To refresh, virtually the only stipulation for September roster expansion is that any player called up must be on the team's 40 man roster. While clubs are being given an additional 15 roster spots, they obviously will not have that many openings on their 40 man. For example, the Twins currently have a full 40 man roster, so anyone promoted that's not currently on it, would make another move necessary.
First, let's take a look at how the Twins can clear some space. There's not a ton of options for the club, but the most likely would be transferring players on the 10-day DL to the 60-day. The problem for Minnesota is that there isn't a ton of options.
Both Adalberto Mejia and Dietrich Enns could be shifted to the longer DL if the club doesn't expect them back. J.T. Chargois has had virtually a lost season, but he's not on the MLB DL, so there's no way to clear a spot involving him outside of a DFA, which isn't logical. The Twins could clear space by DFA'ing players like Buddy Boshers or Hector Santiago, but none of those instances seem entirely likely either. So from a top down view, the Twins are looking at maybe one or two spots on the 40 man being open.
To fill those spots, I'd wager Jake Reed and Stephen Gonsalves appear the most likely. The Rochester Red Wings are currently battling for a playoff spot, and with the likelihood that both would be used out of the pen for Minnesota, I can't imagine a promotion prior to the Triple-A season coming to an end. Reed can definitely help the big club in relief, and should've been up a long time ago if not for an injury to start the year. Gonsalves is going to be a difference maker in the coming years, and while his value is as a starter, getting his feet wet in 2017 is hardly a bad thing.
Looking at what's currently available on the 40 man roster, there seems to be a few more candidates worthy of a promotion. Boshers seems much more certain to rejoin the big club than to be DFA'd, and both Aaron Slegers and Nik Turley should join him. Slegers could be a spot starter down the stretch, while Turley profiles well in relief. Despite being on the 40 man and previously making the jump, I'm not sure there's much allure to bringing either Felix Jorge or Randy Rosario back up. Like Gonsalves, Fernando Romero is an impact starter for the Twins future, but with a workload well above previous seasons, and slowing of late, I'd just call his season quits.
From a position player perspective, there's only two players not on the active roster that are possibilities. Engelb Vielma is a glove first shortstop in the same vein as Ehire Adrianza. With Adrianza already used sparingly, and Vielma hitting just .212 in 84 Triple-A games, I can't imagine it worthwhile to ask him to sit on the Twins bench. From a bat perspective, Daniel Palka makes some sense. He's an outfielder by trade, but survives as a slugging hitter. He's a lefty, not the righty that the Twins need, but he owns a .272/.324/.428 slash line in 78 games for Rochester this year. He's a power threat, and adding another pinch hitter for the final month is hardly a big issue.
The way I see it, Minnesota has very little space to create room, and they don't have a ton of candidates needing a call up either. I think we see more players return to the big league club, than we see fresh faces. Also, despite the big league club taking precedence, I'd be far from shocked if we don't see a few players hold out at Triple-A until and Postseason run is concluded.
If I had to handicap things right now, here's who I see coming up, and the likelihood that they do:
Buddy Boshers 100%
Nik Turley 100%
Aaron Slegers 80%
Jake Reed 60%
Stephen Gonsalves 55%
Daniel Palka 50%
