This is quite easy to implement and It will make every live user very happy.
Just add 5 to 10 buttons with only number on them somewhere near the crossfader so that you can save and recall multiple Crossfader settings.
Also Add an envellope option in the master channel so that you can
record live the A/B Crossfader setting and reEdit it as an step envellope.
(like the mute button)
You would need to add also a lock on those button so that when you crossfade you don't by accident hit any of them.