Begin to browse for other music to learn and develop your playing. The internet and youtube have many samples of players and pieces.
The list is from Gail Robertson's clinic at American Band College 2015. Here is a sample from Youtube of Gail Robertson performing

From the Shores of the Mighty Pacific - Herbert L. Clarke Gail Robertson (euphonium) and Hyekyung Lee (piano) D.M.A. Recital October 15, 2011 - Live Performance Michigan State University and link to my youtube playlist for you to enjoy

 

 

Solos for Beginners

• Air Noble by Jacques Robert

• Conqueror by Leonard B. Smith, arr. Leonard Falcone

• Happy Song by Edmund J. Siennicki

• Pied Piper by Forrest L. Buchtel

• The Rooster by Edmund J. Siennicki

Solos for Intermediate Players

• In the Hall of the Mountain King by Edvard Grieg, arr. G. E. Holmes

• Minuet in G by J.S. Bach, arr. Ronald C. Dishinger

• Minstrel Boy by Forrest L. Buchtel

• Polovetsian Dances by Alexander Borodin, arr. Forrest L. Buchtel

• Sparkles by Floyd O. Harris

• The Young Prince by Floyd O. Harris

• Asleep in the Deep by Henry W. Petrie, arr. Harold L. Walters

•  

More advanced:

• Brass Bangles by Floyd O. Harris

• Carnival of Venice by Henry W. Davis

• Evening in the country by Bela Bartok by Floyd O. Harris

• Honor and Arms by Frederick Handel, arr. Allen Ostrander

• The Jolly Peasant by Robert Schumann, arr. G. E. Holmes

• March of the Marionette by Charles Gounod, arr. Harold L. Walters

• Ocean Beach by Floyd O. Harris

• Pavane Pour Infante Défunte by Maurice Ravel, arr. Ronald Dishinger

• Red Canyons by Clair W. Johnson

• Toreador’s Song form Carmen by Georges Bizet, arr. G. E. Holmes

•  

Solos for Advanced Players

• Arioso (From Cantata No. 156) by Johann Sebastian Bach, arr. H.R. Kent

• Concert Aria by W. A. Mozart, arr. H. Voxman

• Concert Rondo (k. 371) by W. A. Mozart, arr. Jay Ernst

• Fancy Free by Clay Smith

• Fantasy for Trombone by James Curnow

• Mirror Lake by Edward Montgomery

• My Regards by Edward Llewellyn

• Prelude and Minuet Arcangelo by Corelli, arr. Richard E. Powell

• Rhapsody for Euphonium by James Curnow

• Rondo by W. A. Mozart, arr. Ronald Dishinger

• Romanze by Gustav Cords

• The Bride of the Waves by Herbert L. Clarke

• Tramp, Tramp, Tramp by Edwin Franko Goldman, arr. Theo M. Tobani

•  

More advanced:

• Allegro Spiritoso by Jean Baptiste Senaille, arr. Leonard Falcone

• Andante et Allegro by J. Ed. Barat

• Andante and Rondo by Antonio Capuzzi, arr. Philip Catelinet

• Annie Laurie by Arthur Pryor, arr. Robert Geisler

• Beautiful Colorado by  Joseph De Luca

• Carnival of Venice by Herbert L. Clarke, arr. Arthur Brandenburg

• Concerto (for Trombone) by Nikolai Rimsky‐Korsakov

• Concerto Rondo (Bassoon concerto mvt. 3) by W. A. Mozart, arr. Richard Fote

• Dance Suite (Unaccompanied) by Brian Isreal

• Grand Concerto by Friedebald Grafe

• Introduction and Dance by J. Ed. Barat, arr. Glenn Smith

• Phantasy Piece Op. 10, #2 by Burnet Tuthill

• Minuet‐ Scherzo by Joseph De Luca

• Romanza Appassionata by Carl Maria von Weber, arr. P. X. Laube

• Scene De Concert by Max F. Denmark

• Solo De Concours by Paul Veronge de la Nux

• Sonata by  Johann Ernst Galliard/Karl Heinz Fussl, arr. Keith Brown

• Sonata in F major by Benedetto Marcello, arr. Allen Ostrander

• Sonata in F minor by Georg Philipp Telemann, arr. Allen Ostrander

• Suite for Baritone by Don Haddad

• Toccata in the Style of Frescobaldi by Gaspar Cassado/Keith Brown (tenor clef)

• Starlight (Waltz Caprice) by Arthur Pryor

• Variations on a Theme of Robert Schumann by Robert Schumann, arr. William Davis

•  

Solos for Professional Players:

• Allegro et Finale by Eugéne Bozza

• Andante et Allegro by J. Guy Ropartz/A. Shapiro

• Believe Me, If All Those Endearing Young Charms by Simone Mantia/David Werden

• Blue Bells of Scotland by Arthur Pryor

• Carnival of Venice by J. B. Arban/Edwin Franko Goldman

• Concert Fantasie by Gustav Cords

• Concertino Op. 4 by Ferdinand David

• Concertino #1 in B flat Major by Julius Klengel/Leonard Falcone

• Concerto Mvt. 1 (K. 191 for Bassoon) by W. A. Mozart/Allen Ostrander

• Euphonium Concerto by Joseph Horovitz

• Fantasia by Gordon Jacob

• Fantasia di Concerto by Edoardo Boccalari

• Introduction and Polonaise by Max Denmark

• Lyric Suite by Donald H. White

• Morceau Symphonique, Op. 88 by Alexandre Guilmant/E. Falaguerra

• Pantomime by Phillip Sparke

• Ricercare #1 (Unaccompanied) by Andrea Gabrieli

• Sonata for Euphonium by David Uber

• Sonata in C Major by Johann Friedrich Fasch/Fromme

• Symphonic Variants by James Curn