For something written to improve technique I suggest that you consider one or all of the following:
J.S. Bach wrote "Two- and Three-Part Inventions" (thirty short keyboard pieces) for his students.
...with midi samples:
Since you mentioned you have learned Bach's Prelude in C Major why not continue with the remaining 47 Preludes and Fugues from his "The Well-Tempered Clavier"? After all, Bach had intended this for "...musical youth desirous of learning, and especially for the pastime of those already skilled in this study."
By all means let Frédéric Chopin be your guide with his Études. 27 pieces that redefine technique for this period. Chopin's Prelude in E minor should be cool for your next one too, while also you might want to consider Beethoven's first movement of The Piano Sonata No. 14 in C-sharp minor "Quasi una fantasia", Op. 27, popularly known as the Moonlight Sonata.
(Late Romantic era--exercises)
The Virtuoso Pianist (Le Pianiste virtuose) by Charles-Louis Hanon, sixty exercises meant to train the pianist in speed, precision, agility, and strength of all of the fingers and flexibility in the wrists.
Béla Bartók's "Mikrokosmos" 153 progressive piano pieces in six volumes written between 1926 and 1939.