There is nothing more valuable than the Hanon exercises; I would especially recommend the five finger stretched diminished 7th and dominant 7th exercises in the 2nd section. I like mixing patterns together with harmonic combinations of each, a simple example being Cdim7 to B7, in four octaves, and perhaps placing runs of the adjoining major and harmonic scales in between somwhere. The only reason I am telling you this is that because this exercise IN PARTICULAR drastically improved my left hand's accuracy, reach, and touch; before these drills, it was also clumsy and felt mentally detached from my brain. You might try taking the first 8 measures from Chopin's Fantasie Impromptu (a nice left handed stretch though a basic minor progression) and working it through the circle of fifths in a way you see fit (muddy or not). Those sweeping left hand arpeggios in all keys will get you there, if you are not madly bored from something like that. It makes for good practico. Do whatever you like with your right hand...