I don’t see why these features would need a major version update…
Regarding VST plugins: I’m not sure if your’re talking about scsynth hosting VST plugins or scsynth as a VST plugin.
The former has been possible for a couple of years now (VSTPlugin v0.5.1 - bug fix release!). I’m pretty sure it is better kept as an extension.
For the latter we only need to add a generic “plugin” audio driver (similar to the existing AU driver, but not restricted to a specific audio plugin API) and then people can build VST plugin frameworks with libscsynth, just like people already do with libpd (Pure Data). Actually, scsynth already works as an AU plugin, so there is no reason it couldn’t work as a VST plugin as well.
and a similar issue exists for VST that also involves licensing related complications.
The VST3 SDK and SuperCollider both are licensed under GPLv3, so there wouldn’t be any issue.