D minor piece

Still working on it...

I think it's clever that the end of each of the triads functions as both as a pivot tone and an extension, however, I feel like being restricted to triads in the main bulk of each chord is restricting and can get a little repetitive. 
Speaking of repetitive, you're going to want to change your rhythms up. It's too much of this right now. No contrapuntal line, or really a melody for that matter; just a bass line, that goes to the treble staff. Repeated time is just too long. Doing this would also eliminate some of the monotony of the chord progression as well; it'd make it more tolerable so it's not noticeable you're using the same one each time.
 

I agree that if your main interest here is the harmonic progression, then there has to be a change in the rhythms. It is possible to have an engaging piece but only out of the same rhythm repeated again again (The first two Bach preludes, for instance) but for that to work there would have to be more harmonic interest or at least some sense of a melody. 

What you have here is pretty, it's just too much of the same.