I'd like to see a large shift from taxing productive activity to taxing things like pollution. I.e. reduce taxing things we want people to do and increase taxing things we want less of.
The result was a massive shift out of relatively unproductive tax shelter investments into more productive ones. Although I've never seen this opinion in print, I suspect a lot of the economic growth that resulted was a consequence of this shift in investments.
In order for this to work, we would need to include a way to tax assets as well, as otherwise people could just put their money into assets.