For example, if you have a lot of vids in directory with more/less similar content and thumbnail, it is hard to understand which are already added and which are not. Maybe added vids thumbnails could be marked with a star (cross, dot, shaded)?
As well would be nice to stick project to specific folder (last used folder), because in case of many folders with video if is not really convenient to navigate to required folder each time you need to add another video.