It would be nice to click anywhere on the screen in order to capture a preset size region centered on the very same point you clicked.
Example: let's say you are looking at a logo an you want to capture it for a blog post. You could define a specific width*height as a preset and activate it by pressing Ctrl+Shift+click on the center of the desired region.
Or, as an alternative, the drag feature could work from the center instead of one corner, following a width:height ratio.
I just noticed this feature is pretty common so it shouldn't be hard to implement. It's called "fixed region" on other capturing programs.