A simple QoL addition. Just like you can search usernames in PMs to find all your chats with that user, a similar addtion would be on notifications, showing you all notifications connected with the person you searched. This would also give opportunity to add some analytics, for example a ranking of top 5 users who gave you most notifications. The whole concept would be useful since scrolling too far back into all the notifications triggers an error 1015 that temporarily kicks you out of the site.