If you want true melody taking over percussion, you're going to have to use a mallet instrument....which somehow I don't think the assignment meant...so I'd forget that.....
If you want something with different pitches, timpani was a great suggestion (it does sound awesome with trombones)-you can write for 4 (5 if you're brave or a rich school is playing the piece) drums tuned to pitches. Keep in mind though that changing pitches on the same drum does require a few beats of rest... and very few players could pitch by ear on the fly accurately. Give the poor timpani player a break (can you tell what I've played?
). However, a timpani can do a cool glissando (up sounds better)...
Rototoms might be interesting, can be tuned, but if this is being written to actually be played I'd stick with timps. If you're looking for something different, latin percussion may not have a definite pitch but there's several drums so some semblence of a melody *could* be done. (congas, bongos, etc).
which brings up my question. What pitched percussion would blend nicely with 4 trombones?
i started writing with tubular bells but i might change to marimba. 
