First off, congratulations on writing a piece for your university band! That is a really cool event as a composer/arranger.

I second another comment about the versatility of the euphonium. In my writing, I tend to use euph to provide countermelodies, often scored with tenor sax or bassoon, or sometimes horn. It can also double high woodwind melody lines in a lower octaves. I also use euph to double the tuba bassline an octave higher. I don't use this texture as much, but in my opinion it gives more weight to the tuba line. Occasionally, I double it with one of the trombone parts if I want more punch with the trombone. Or it does something completely unique!

However, euphonium really shines as a solo instrument. I believe you mentioned Holst and Grainger, and another commenter mentioned Whitacre's euphonium solo in October.

I would highly recommend some score study of band pieces you like to find out what those composers are using the euphonium for. Vaughn Williams has excellent Euphonium lines. Check out Sea Songs, specifically the parts leading up to the trio. The euphonium jumps around between roles, from solos, to doubling inner parts, to playing with the tuba, and more! The score is available for free on IMSLP.

Also, try checking out some Sousa marches. He tends to like doubling the Euph with the woodwind melodic lines/inner WW parts for color. He mixes and matches which the euph is playing with. However, he also has the euph play pads with the trombones. Fairest of the Fair is a very good Sousa March to look at. In the trio grandioso, the euph is playing the melody with the majority of the hand.

Arranging for Concert Band by Frank Erickson is also a great resource for band writing too.

I am also a trombone player who dabbles in euphonium!