I have been using KDE and Spectacle for many years and this is a feature I would find useful: When taking rectangular screenshots, I would like to be able to lock the aspect ratio. There could be presets of common aspect rations 1:1, 4:3, 3:2, 16:9 and then user custom aspect ratio. When a fixed aspect ratio is selected the rectangular region would be constrained to this ratio, so only move and resize is possible.
A next logical and in my opinion useful step would be to allow the user to select a fixed size for example 800*600 pixels for their screenshot. In this case both dimensions of the rectangle would be locked and only the position could be changed.
I sometimes want a screenshot of a particular size and it can be done by manually resizing but pixel level accuracy with the mouse takes a bit of effort.