Trombone quartet

I've been playing trombone for several years now; this is my first serious attempt at writing for the instrument.

Measures 1-6 introduce the listener to the polyrhythm.

Measures 7-14 is the opening period. I want to eventually flesh this out into a substantial A-section.

I have not started a contrasting section, but measures 15-16 are a climax into a renewed A-section.

Any ideas for developing the opening further? I'm also looking for texture-related suggestions so that the 4th trombonist isn't bored out of their minds. 😛

My only idea is to double the melody in thirds for the renewed A-section.

I'd say... change the rhythm.

It's not complicated enough to stick to it for the whole piece.

Add another rhythm, and then you'd be able to mix them in several ways.
The new A doesn't have to be exactly the same.

Change it a bit so it'll still be interesting.

Example: