For the Tie button (and shortcut) to not only tie, but also un-tie.
Basically that. Currently the only way to remove a tie is to select it with the mouse (which is rather cumbersome because it's a thin line) and press delete.
If there are already a button and a keyboard shortcut that tie the selected note when it's not tied already, wouldn't it make sense for both this button and shortcut to also un-tie the selected note if it happens to be tied?
EDIT: The button and shortcut for dotting and double-dotting notes work the way I describe, so it's also a matter of consistency.