Thank you responding, @J. Lee Graham! I suppose I should give some introductions to my list, as you have done with yours.
Viktor Kosenko (1893-1938) was a Russian-born Ukrainian-Soviet composer and pianist. He wrote for almost every genre of music, including concertos and piano music. He is probably best known for his Passacaglia in G minor for piano, a magnificent work of epic proportions. Even though he was around during the Soviet era, he mostly wrote in a late-romantic manner. It's wonderful stuff to listen to.
Toivo Kuula (1883-1918) was a Finnish composer. He wrote mostly songs and chamber music, and has an unfinished Stabat Mater. Personally, I love his massive Piano Trio in A major, particularly the 3rd movement. It's some very emotional and passionate music. Unfortunately, he was killed at age 34 from a scuffle with a drunken soldier at the end of the Finnish Civil War, from a gunshot wound to the head.
Wilhelm Reinhard Berger (1861-1911) was a German composer, pianist and conductor. He was a very prolific composer, have completed over 100 opuses, although much of it remains unperformed. I consider his Piano Quintet in F minor to be among the best piano quintets ever written, and, like Kuula's Piano Trio, is a huge work, lasting roughly 50 minutes. A criminally underrated genius, in my opinion.
Giuseppe Martucci (1856-1909) was an Italian composer, pianist, and conductor. He was the first Italian composer in decades (if not centuries) to not write an opera. His output includes 2 symphonies and piano concertos, and much chamber and piano music. It's very sophisticated music, and is definitely worth checking out.
Vasily Kalinnikov (1866-1901) was a Russian composer. His life was a tragic one, to say the least, having been impoverished most of his life, and dying at aged 34 from tuberculosis. Probably his best known works are his 2 symphonies, both of a fresh and magical quality, with touches of Russian nationalism incorporated in the music.
Wladyslaw Zelenski (1837-1921) was a Polish composer, pianist and organist. If at all, he is probably best known for his chamber music, including his Piano Quartet in C minor, an excellent work, that I would highly recommend to any chamber music-lover.