I'm thinking in a simple approach with the Timefactor in mind. I'm pretty sure that a lot of people might want a more "real" volume pedal embedded in the Stompbox by controlling it with an Expression Pedal.
It will take a simple switch stomp, similar to "rpts", to activate and deactivate.
User can select either (rpts or swll) to use in "Play mode" by pressing the middle switch.
Decay time, from 0 to 10+ secs, set by the user in the globals menu.
MIDI, sure, why not.
Make it global.