I think it would be interesting to have an option within the webadmin to display a message on all sites and services across the server to show important messages such as planned downtime for maintenance.
Yunohost already has something similar: the yunohost overlay that is (optionally) shown on all websites and that, if clicked, takes the visitor to the SSO area. My ignorance makes me believe that the banner could be similarly implemented to show an alert on top of the screen to warn users about an upcoming event without using email.
The content of the alert should be editable and maybe there could be a way to schedule when it should be displayed (eg 72 hours before the h hour) and when it expires.
Like when you pin a topic on the forum.
What do you think?