Love this idea! (Although I too would have a difficult time setting aside my Pro-Q 2 if we stuck with Sonar-only plugins, but that certainly makes for a more interesting competition, not to mention a level playing field!)
Maybe raw tracks for a different genre every month, or whatever timeline is established? And Olympics-style Gold/Silver/Bronze awards for the top three vote-getters so the community can see how differently the same tracks could be mixed by different people?
I also like the idea of adding an element of arrangement. Maybe for each song, there are two categories: one being a straight-up mix of only the tracks provided, and another allowing for additional instrumentation, background vox, etc.
Anyway, again, great idea. I definitely agree that some friendly competition will help remind the community of why we're here in the first place - to make music - as opposed to the tendency to get caught up in discussions about bugs, missing features, etc.
Definitely going to be tracking this thread!!