It would be nice if we could create one or more areas bounded by splines that could be used as a Garbage Matte. Everything bounded by the splines would be forced to be invisible. This would make it possible to have a green screen that does not fill the frame. Also, you could put markers on a green screen that don't get chroma keyed out for use as motion trackers. The garbage matte would be an easy way to force areas to be invisible and let the images behind them to show through.