Not sure if this has been requested, so please excuse me if it has?
Would it be possible to have a "Crop Area Tool" that could be set to "Free Transform" ? Many times We re-render select areas of Our Scene, and many times it's an area that is diagonal across the Scene. With the square/rectangle that We have now, We are forced to render areas that We really don't want to re-rendered. This would save wasted render time and energy. Hope I'm explaining this clearly?