If we are on the subject of examining and studying repertoire:
"For us musicians, Beethoven's work is the pillar of cloud and fire which guided the Israelites through the desert- a pillar of cloud to guide us by day, a pillar of fire to guide us by night, so that we may progress both day and night" His italics.
Now here were some works and what I would look for in these works:
Theme construction: Schubert Lieder, Beethoven, Chopin Sonata
Harmonic construction along with theme:
Liszt TEs, Beethoven, Rachmaninoff concerti, Schubert sonatas, Scriabin. Tchaikovsky especially-his harmonic control is incredible (orchestral works as well! They are very influential).
Textures (IE vocabulary):
Alkan Symphony & Concerto, Festin. Chopin Etudes, Liszt Etudes, Scriabin Etudes. Also, orchestral works, such as Wagner (!), Tchaikovsky, Mahler. The orchestral idiom was highly influential on the style of Liszt and Scriabin.
Fugal/imitative development:
Bach Fugues, Art of the Fugue
Thematic Transformation:
Brahms Miniatures, Liszt, Cesar Franck, Beethoven.
Variation development: Brahms miniatures, variations serieuses, Bach-Busoni Chaconne, Rachmaninoff Rhapsody, Beethoven 32 variations in C Minor, Eroica variations, Brahms Handel and Paganini Variations.
Counterpoint: Godowsky Passacaglia, Brahms, Beethvoen, Mozart. Use the guide above.
Large-scale structure: Brahms large works, Beethoven Large Works, Liszt Large Works, Other symphony, Brahms Chamber Music.
Variations for effects:
Experiments in impressionistic effects, ie Liszt Harmonies du soir
Stretching form or surprising the audience when they expect something different
Dramatically change harmonic progression upon variation or repetition
Orchestral textures fitted to the piano: As above.
Motivic development: Liszt Sonata, Beethoven, Chopin B-flat minor sonata (Note that here Chopin succeeds, once again, in forming a cohesive whole out of seemingly jaunty or rough parts.
Further suggestions/areas?
I have, personally, decided to do this: work on series of different pieces as practice, and when comfortable, right 24 characteristic pieces of various characters, one in each key. These would be preludes

. Then after that, some further works, eventually building up to a Sonatine A la Haydn, then a Sonata A la Haydn then Sonata a Early Beethoven. After that, I plan to make two sets of 12 Etudes, one being simple etudes to explore the keyboard and possible innovations and expansions and resources, as well as a testing ground for harmonic development. Then, a large set of 12 Etudes, further exploring all aspects of piano composition and being of great musical value. These finally would culminate in the ultimate creation, the Grande Sonate (Don't know what language it'd be in lol.) I plan for it to have the sheer power of grandiose works, yet the structural and musical integrity and form and intensity of emotion therein of Brahms, or to create the most moving and powerful work that I could possibly create.* Now, to phil, note that a lot of Scriabin's chord progressions were chromatic variations or augmentations of the standard chord progression, suggesting further area of exploration.
*P.S. Borodin was a doctor and was what he described a "weekend" composer.
I hope I don't kill you with fright.