KRecentFilesAction (https://lxr.kde.org/ident?_i=KRecentFilesAction) is used in many applications. It would be nice if it was possible to add a Pin button for each file entry. Pinned file entries would always be on the top of the list and will not leave the list as long as the Pin "is on" (except the list is deliberately cleared). I use this kind of feature with MS Word and it turns out to be very helpful when dealing with a lot of different files but a certain subset files are regularly used and would otherwise be pushed out of the recent file list by the other files. (This could later also be reused in the task manager to pin open documents which would appear in the jumplist)
A recent planet.kde post in which mentions the usefulness of being able to pin recently used documents: http://cukic.co/2017/08/17/plasma-shelf/
+1, I like this idea. See also https://phabricator.kde.org/T9041