I think many Blender tools and addons could benefit from a dual selection system. First you select a bunch of elements as a source (vertices, edges, faces or objects), then select other elements for destination (with a different color).
Right now, you can only do one single selection and, at most, combine it with the active selected item or with the 3D cursor and hope for the best that the algorithms will guess what you want to do.
The active selection and the 3D cursor act as a second selection already, but what if you need to have a secondary selection of more than one element? There is currently no way to tell Blender where to start and where to end some complex operations. It’s like using a hand and a finger instead of two hands, I find this to be an unnecessary limitation in my opinion.
From a technical point of view, having a dual selection will create an opening for new tools and addons to kick in and do a lot more than the current tools.