At the moment, if you access Special:Notifications on a wiki, you will have the list of local notifications (read and unread) and cross-wiki notifications (unread).
As far as you haven't read all cross-wiki notifications, you can browse any cross-wiki notification, whatever their status. When all cross-wiki notifications are read for a give wiki, you can't find any of those cross-wiki notifications for this wiki. You have to go to that wiki where was the notification to have access to it.
So Special:Notifications should be more wiki-agnostic by displaying the list of wikis you've received notifications from every time, whatever their status.
* The local wiki notifications should stay be pinned on top.
* The order to display cross-wiki notifications could be
# All wikis with unread notifications, by order of unread notifications (the more, the upper),
# All wikis, by order of notifications received (the more, the upper).
* While we have a thousand of wikis, a search bar for wikis would be a nice bonus.