If unexpected note occured, program may play it using instruments that are currently active in this area of keyboard or it may ignore it and dont relay signal to VST. So user may choose between 2 tempting options: either program will filter out wrong notes, or itll let you improvise and wait until you get back to expected material.
Technically its simple: there should be an option of creating traditional "solid rig" of instruments inside a project. In this mode itll stop reading the score and start working as MainStage or Cantabile. This rig should be available at any time during performance by pressing special key or MIDI trigger.
For more convenience there should be “improvisation cue” feature, which will let user define a group of notes that will serve as a keyswitch to “solid rig” mode. Another group of notes played in a row during improvisation will switch program back to “reading score” mode. So its like working with a live jazz band - players listen to you while you improvise and know that if you played "these notes", its time to catch up with you 
Actually I already made a concept with all main features and interface ideas. Wondering how to deliver it to Steinberg or any other software developer. They all don`t have any contacts for that kind of proposals, in public access at least. Any ideas? 