Evan Grant has a blog post up that looks at how signing Jim Thome could impact the Rangers' roster, lineup, et al.
Grant suggests that signing Thome would mean Michael Young taking 40-50 PAs apiece from Elvis Andrus, Adrian Beltre, and Ian Kinsler, with him playing the field a lot when Thome DHs, and with Young and Mitch Moreland each losing close to 100 PAs as a result. I don't see the Rangers playing Young at shortstop, though, so I don't know that that's an accurate number.
Grant also breaks down where the bench likely stands right now without Thome, and lists Chris Davis as a possible 25th man as a defensive specialist, which I have to think has no chance of happening. The Rangers aren't going to waste a bench spot on a third string first baseman, particularly given that Davis is someone who could have a future in the league, and if he's going to have a future, should be in the minors getting regular ABs, not coming into the game to play defense in the 9th.
Craig Gentry or Endy Chavez probably are the primary candidates for the 25th man spot right now, as either of them could play centerfield against lefty pitchers and pinch run.
I've still got issues with how Thome would fit on this roster, given how it is currently constructed. Thome really doesn't fit, I don't think, unless you are going to have him replace Mitch Moreland on the roster, and have Moreland start the season in AAA, with Young being your everyday first baseman. Of course, if you do that, that eliminates the roster flexibility and ability to use Young at multiple infield positions, which was supposedly the plan.
And it also means that you've still got to go find a righthanded hitter to split time with Thome at DH or to play first base against lefties so Young can DH, since you aren't going to have Thome play every day, and you presumably don't want David Murphy to be your primary DH against lefthanded pitching.
For all the carping on the twittersphere about the Rangers being too righthanded-oriented and needing Thome because they need to be more lefthanded, people seem to be ignoring the fact that their bench and part-time players are mostly lefty hitters. Adding Thome makes David Murphy useless, since he's not going to play against righthanders, and you don't want to play him against lefties. Which is what prompted my comment about trading Murphy for Marlon Byrd, although the Cubs probably wouldn't do that.
The Rangers' bigger issue, it seems to me, is that they are vulnerable to lefthanded pitching as the roster is currently constructed, and adding Thome makes that worse, not better. Yes, adding Thome means that you've got a fallback position if Moreland struggles, but it still means that you've got a problem with your lineup against lefthanded pitching.
If the Rangers sign Thome, it doesn't really solve their roster construction problems, and it puts them in the same situation they were in last year, where they've got a lack of flexibility with their DH spot, a player who can't field taking up a spot on the roster, and too many lefties in the lineup against LHPs.