It would be nice if power users could modify their Echo notifications on the client side. For example, the link notification does not link to either of the main pages in question, just Whatlinkshere. Power users may want to add these links.
This is more complicated than a typical user script since the notifications are fetched from the server every time you show the overlay.
Adding a hook for after notifications are shown would provide a user script extensibility point. This should fire after the overlay is built and after Special:Notifications is shown, so both can be tweaked the same way.
mw.hook (https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.hook) seems like a good tool for this.