Selection / Crop tools: Move some controls to a canvas overlay
Of the tool options in GIMP, the Position and Size options for the Rectangle/Ellipse Select and Crop tools feel like some of the more awkward controls there, squeezing a number of widgets into a tight space and still taking up a lot of it in the process. As well, I don't know of any use for changing these controls outside of editing an active selection/crop, since doing that will just get them overwritten the moment you interact with the canvas to start a new selection.
All this considered, I think these controls would be perfect to move into their own canvas overlay, like those used by the various transform tools. This would both de-clutter the valuable space of the Tool Options dockable, and give the controls a more liberal area for their widgets to be more conventionally laid out, all without any loss of functionality.
Some other bits could be added to the overlay as well. Moving "Auto-Shrink" over comes to mind, itself also only useful when editing an active selection. Further conveniences could be buttons for fitting the interactive box to current layer/canvas, floating/duplicating a selection, or cropping from a selection.