Concert Band Piece

I'm wondering what to do next and what to change.

Hi! I think you have a good start going here. I especially like your opening passage.

Generally speaking, I think your melodies here are tending to be very step-wise and linear in a single 'direction' up or down; some skips and leaps can add interest and excitement, particularly when they're used to change the direction of the melody. As for what to do next, I think it would be good to transition to a new section with a faster tempo to build some excitement. Since we're talking about a concert band, it would be great to hear some percussion get involved -- preferably both pitched and non-pitched. Just some thoughts. Well done so far.

Arpeggiated F minor triads near the end sound awkward.
Be careful of the inversions of your chords, especially when the bass voices move on their own. Triadic content needs to match on top.