In companion to "Audio Out" there could be a new "Render" module which the user can make several of and wire individual contraptions/vst up to each one so that when it comes time to render the track each Render module gets its own wav saved to disk.
This will make it possible to create stems for importing into Logic and Cubase and such for mixdown.
Yeah, I've been thinking about some way to do multichannel bounces, possibly just using the AuxOut contraptions in combination with the existing Export to Sound File function.
Right now you can record multiple stems using a multi-channel FileRecorder contraption (e.g. 8FileRecorder). I know it's not exactly what you're asking for, but it will record multiple synchronised tracks.