I know this is a big one, but a very careful and thorough adjustment of the dynamics that each instrument sounds at, at every given dynamic level, would be fantastic. In short balance all the instruments better, which would even mean re-recording some instruments (but not all).
Some instruments are well balanced when playing slowly, but when speed increases their volume drops (clarinet for example). Some articulations are still very inconsistent, better at certain dynamic levels than others, better at certain pitch ranges than others.
Some instruments were horribly sampled (piano, marimba, etc) and need to be done from scratch.