Puccini, Vivaldi, Renaissance and Middle Ages music, ethnic music, Rossini, Verdi, Massenet, Berlioz, Bellini, Gregorian Chant, Paganini.
There are countless composers out there who wrote practically nothing for the piano. I get irritated with any pianist that doesn't know music beyond Chopin, Liszt and Rachmaninoff. A lot of the best piano music out there is inspired on these composers that many pianists are not familiar with--- Bach transcribed dozens of Vivaldi concertos and continuously imitated the Italian baroque forms in his music; Chopin idolized Bellini and his groundbreaking way of writing for the piano was an attempt to imitate the singing melodies of Italian opera; Liszt was mesmerized by Paganini's diabolical virtuosity and set out to create that same kind of sound for the piano; practically every composer tried to imitate some kind of folk music elements from his region, and that kind of music in its raw form is easily obtainable today.
The hardest thing for pianists to do is playing a good melody, what better way to learn that than by listening to music for the voice? Even so, I'm surprised that so many "pianists" can't name more than a couple of operas and some have never sat and listened to a single one.
Stop being a pianist, be a musician. There is a lot of music out there, don't limit yourself to the clanking of our 88 keys all the time; don't box yourself in. By listening to all kinds of music, your piano playing can only improve.