Miller's Off the Roster
Just as an aside before I speculate on who is likely to get added to the roster in Miller's place, I've noticed that the Twins website doesn't include Chris Basak on the 40-man roster, and neither does any other roster I've seen (on ESPN.com, for instance). However, I've looked at the transactions very closely and I believe he's still on the roster. Basak was claimed off waivers from the Yankees in mid August, and was promptly optioned to AAA. Since that time, there's been no more news regarding his status. That leads me to believe that he is, in fact, still on the roster. Leslie Parker's article on the Twins website (linked above) provides support for this proposition, since it mentions that after Miller's outrighting, the Twins have two open slots left. If Basak wasn't on the roster, then the Twins would have three open slots rather than two. I'll stick by my position, then, until and unless someone shows me a transaction that removed him from the roster.
As for the speculation on who gets added, I think the most obvious candidate is Nick Blackburn. Not only has Blackburn put together a very solid season, he's also eligible both for the Rule 5 draft and for minor league free agency. With the performance that he put up this year, the Twins will want to protect him, so he would have to be added shortly after the season ended anyway. Since spots are available, there's no reason not to add him now and maybe give him a chance to make his major league debut.
If the Twins elect to add another player to the roster before the season is over, the smart money is probably on Jose Morales, a catcher who has had a very solid year for the Red Wings. Like Blackburn, Morales is Rule 5 eligible, and is no longer in the shadows. If he is not added to the roster, someone will draft him, and the Twins almost certainly aren't going to let that happen.
Another possibility is middle infielder Matt Tolbert, who has also been much discussed as a potential callup throughout the year. Tolbert will be eligible for the Rule 5 draft for the first time this winter, so as with Blackburn and Morales, adding him to the roster is very likely. I think his chances of being added before the end of the season are slimmer than Morales' chances, however -- but I would expect that before November, he'll be on the squad.
It's always possible that the Twins could throw a curveball in this area, but I somehow doubt it. Look for one, and possibly two, of the players named above to be added to the roster -- and watch for any players not mentioned next spring.