Orchestra pieces in which the trombone plays an important role
The trombone was often used to play religious music during the classical period. "Tuba mirum" from Mozart's "Requiem" (KV.626) opens with an impressive trombone solo. He is thought to have used the trombone to represent a reverberating trumpet announcing the Last Judgment.
As trombones began to be commonly used in orchestras entering the 19th century, the instruments started to influence the reverberation of the entire orchestra. If you listen to symphonies that require the use of trombones and many other brass instruments (such as Bruckner's work), you'll hear how trombones are sometimes used to create a full and beautiful reverberation, or at other times to produce a solemn reverberation. The first movement of Mahler's "Symphony No. 3" includes some parts that feature impressive trombone melodies, proving that the trombone can be used as a solo instrument within an orchestra.
You can also hear a trombone solo in Ravel's "Bolero," in which solos are demonstrated one after another for the instruments being played in the orchestra.