I hope you don't mind IMRef, but I'm going to update this one a little bit.
Rosenbaum seems to have the best chance to be the next pitching "prospect" to reach the big league level. I've seen Rosenbaum and other Nats coaches mention that Rosenbaum, for some reason, lost command of his fastball/sinker last year. As a result, he developed his changeup and it became an out pitch for him. So that brings his scouting report to something along these lines at the start of this year:
A left-handed pitcher that features a low 90s fastball. He builds off the sinker and will use it to get ahead in the count, but also to generate a large number of ground balls. His slider (might be better described as a slurve) is also a pretty well developed pitch and when it is working on any given day, can be used late in the count to induce poor contact or even strikeouts. The changeup is still somewhat of a work in progress, but has shown the potential to become a plus pitch. He also has dabbled with a cutter and curve over the years. Rosenbaum does not have overwhelming velocity, but has a dominating attitude on the mound and can be crafty with his pitches. While he won't blow the ball by you, he doesn't nibble and rarely walks guys. Has shown the ability to be efficient with his pitches and has the potential to be a back end of the rotation innings eater at the major league level.
So far in three starts, these are Danny's stats for AA Harrisburg:
21.2 IP, 1.25 ERA, 14 H, 0 BB, 16 SO, 2.85 GO/FO, .177 OAVG