Exclusions for clipboard sync
Certain applications I would prefer not to sync the mouse selection and clipboard, e.g. editors that do things like select text when a text box receives focus (I'm looking at you, Geany replace dialog).
More generally, some applications such as editors, or maybe libreoffice, are conducive to using keyboard navigation and selections. It's a minor annoyance when using ctrl-c to copy to the clipboard is subsequently wiped out by a text entry that selects text upon receiving focus.
The ability to exclude an application, by window title, name, or some other property, would be nice.