The sound engine isn't as important as the GUI. I think we're seriously not thinking about what's possible and just keep replicating old tech.
1) Every control on the VSTi should not only have MIDI learn, but do something quick like right click on any controller, and a graphic would pop up to allow a MIN and MAX value be assigned for both the modulation depth and controller value . . . (real quick). . .
Where the left-right arrows would set the MIN/MAX controller value (and as you change it you'd hear the value being played), and the up-down arrows would set MIN/MAX modulation depth.
2) Further, it should allow multiple cc#'s to be assigned on each controller, and each cc# could be assigned to multiple controllers.
3) It would have an easy way to set scripts defining performance-based sound sculpting. For example, let's assume you've assigned Velocity to modify volume, filter cutoff, the amplifier's Attack (longer when playing softer, shorter when harder) and Release (shorter when playing soft, longer when playing hard). But you've also assigned the Note value to also modulate the filter cutoff and the amplifier's Release. You could set the script to apply the modulations from the Note value first, then the modulations by Velocity--or the other way around, or just sum the two. This would allow for extremely expressive performance control over all functions of the VSTi without a lot of fastidious cc automation.
4) Of course the VSTi would also allow you to save scripts and CC assignments in templates so they could be quickly reapplied on other patches. Or they could be set as the VSTi's default. You could even set up the VSTi so that when you start a new sound, you could choose between different default controller groups such as hammered, bowed, plucked, reed, horn, percussive, tuned percussive, etc. and then when as you build the sound, tweak the controllers as needed.
With certain types of synths like FM and Additive, this type of controller assignment and scripting could open whole new realms of harmonic manipulation, while simultaneously making all the complicated stuff go on behind the scenes.
I don't think any of this is beyond current technology, it just needs to be thought through so that setting up the assignments/scripts is intuitive and quick.
