Please add support for overall tempo adjustments, based on existing tempo track, like, an additional box, to say tempo should be 50%, 90%, 150% of current tempo track. This will then scale the entire tempo track by this percentage on output/playback only, retaining the original tempo track values and setup. 100%=tempo as set in tempo track.
Or enable this overall scale tempo on mixdown.
I am not sure the impact on audio tracks, but to have this on projects containing only midi and click tracks, it will be useful to create backtracks to practice at slower (or faster) tempos, so that you are performance ready for track at default 100% scaled tempo (i.e. original tempo track values).