http://www.propellerheads.se/news/rackextensions/
If Vienna Ensemble Pro became a Rack Extension, it would mean you could host VST plugins inside of Reason without the need for ReWire or to wait until your favorite developer jumped onboard. Or if Vienna Instruments Pro became a Rack Extension, Reason users would finally have a quality [native] orchestral library.
Or is there some technological roadblock Propellerhead implemented in their Rack Extension platform that would prevent all of this?