If the Twins are going to extend the quality of the bullpen it will probably have to come from outside the organization. The injured arms coming back may not perform as before (other than Thielbar) as they don't have a track record of major league success.
There are probably only 2 minor league relievers worth a look and thats Kody Funderburk (LH) and Reggie Grace (RH), maybe lefthander Michael Boyle. Everybody else just allows too many baserunners. Keuchel has pitched well enough at St. Paul but is probably ramping up arm strength and endurance.
Also, I don't like the way acquiring other people's arms has turned out for this front office: high price paid with questionable performance and or health in return.
The What-Are-You-Crazy choice would be to promote Marco Raya to throw some late inning darts, but would that interrupt his development?