I don't know if anyone would be interested in this or not....but integrating something like "Band In A Box" where you type in chords and the program plays them along with a selected song type or rhythm section style would be cool for a quick way to put together a new song idea. Band in a box originally was just a glorified rhythm organ, but it has grown over the years. Sonar has all the great synth built in...
Just a thought....

For the time being, you might want to take a look at options like Sundog Scale Studio.
 
http://feelyoursound.com/sundog/
 
For technical reasons, Sundog needs to run standalone and not as a VST synth and connects with SONAR via LoopBe (a virtual MIDI cable). You use your preferred synths/VIs in SONAR.
 
http://www.feelyoursound.com/sundog-with-cakewalk-sonar/
 
You can demo it for free. The full version is $45, which I think is very fair. It works for me; maybe it will work for you!
 
Here's an example of one of their tutorials.