There is loads you can do. All your lateral work can be done in walk and should be mastered in walk before you try it in trot. I would do some leg-yielding, turn on the forehand and shoulder in, this could be done both in school and out on a hack. You could also to a 20m circle that you spiral in and out using leg-yielding. I would also work on flexion and counter-bend to increase your horse's suppleness. Hope that helps.