In a perfect world, I’d love to see the ability to attach a volume pedal, which could be used as a tempo control.
Upon the start of a song, the song’s default tempo would be used, with the volume pedal’s current position being tied to that tempo.
If you started a song out with pedal fully down, the song’s default tempo would be used as the default and maximum tempo for the song. Pressing back on the pedal would slow the tempo down.
If you started a song out with the pedal fully up, that would be the default and minimum tempo for the song. Pushing down on the pedal would speed the temp up.
If you started a song out with the volume pedal set to halfway, pushing down on the pedal (‘giving it the gas!’) would speed up the tempo, and “hitting the brakes” on the pedal would slow the tempo down.
There might need to be a “sensitivity” control in the firmware to allow for setting the overall amount of tempo change applied.