Add a parallel slot in the existing "FX Chain" plug-in where two vst can be placed in parallel.
A balance control would be added to control the volume balance between the two vst.
If one half of the parallel slot is unpopulated with a vst then it will be a unity gain pass-through (but still affected by the balance control).
 
The purpose of this would be for a simpler way of doing "New York Compression" when using compressors that don't have a mix control, or for special effects, IE: two different compressors in parallel, two different effects in parallel, etc.
 
I currently do this using busses but large projects tend to get unwieldy having the extra busses.