The starting point for callups is players already on the roster, since players called up have to be on the 40 player roster. While players can be added to the roster, recent draftees are rarely are until they need to be protected from the Rule 5 draft. This is why, even before his injury, it was unlikely Kyle Gibson was going to be a September callup. I think the same is true of Liam Hendriks. One player who is not on the roster now, who I think is very likely to get a callup, is Carlos Gutierrez. He has to be protected from the Rule 5 draft and he is a reliever which means there will be plenty of opportunities for him to play.
There are four categories of players who often get called up by the Twins. One is minor league veterans of the Twins system who are getting rewarded for their performance in the minors. I don't think there is anyone who really fits that profile this year. The second category is players who have been in the major leagues, but got caught in the numbers game and sent to the minors. I think we can expect to see Luke Hughes, Rene Rivera* and Jason Repko back in the big leagues. Although Repko's spot may depend on having outfield at bats available and Denard Span's health. The third category is players who provide depth, but that is more likely when the team is in contention. When the team is out of contention, those spots are more likely to get filled by the last category, young players the Twins want their manager and coaches to see in major league competition. This year, that list is probably pretty short, since most of the young prospects who are ready for that test got the opportunity during the regular season.
For everyday players, Chris Parmelee is a possible callup if they think they can provide at bats at first base. This probably becomes more likely if Thome is sent to a contender since that would allow Morneau and Mauer to DH more often. Its also possible they will want to see Joe Benson try to hit major league pitching. But the outfield already looks pretty crowded. Brian Dinkelman seems like a long shot for the same reason.
On the pitching side there is a long list of possible callups. In addition to Guttierez, I think Scott Diamond is a very likely callup. I would also not be surprised to see Deolis Guerra called up. He has been pitching out of the bullpen and is out of options after this year. I think they will want to see the guy they got for Delmon Young, Lester Oliveros. And they may also want to see more of Jim Hoey. Chuck James is another guy they may call up. They need to decide whether he is worth a roster spot this winter.
The players I really want to see are Guttierez, Guerra and Parmelee. They are the three players with upside who may contribute to the Twins over the next five to ten years.
*correction: Rene Rivera, not "Ben Revere" as originally posted.