The BBPlugin is an ideal place to start as it uses crossplatform JUCE that is released at the same time on Win/Mac and could be on Lin.
Keep the Win and 6 months later Mac released how it is but get the generate code into the Plugin, this will give instant generation of RealTracks/Drums, faster than 90's way Biab works now.
It will give you a Plugin, Plugin Standalone and a Live Arranger. If I could get the RealDrum generate code into Reaper PG could certainly get the RealTrack generate code into the Plugin. Once this is done other functions can be added as they go if need be. In the long run I think you will see the the current WinBB then 6 months+ later released MacBB phased out.