Instance-only admin/notification notices
They could be some kind of local-only post that goes to all users on an instance (regardless of whether they are following the admin or not) on the top of their TL. The main use for them would be to announce downtimes and upgrades and the like.
Since this is an instance-only thing, there's no necessary reason to match how it is done in other software. However, it may be useful to see how Masto does this if it catches on over there for issues like API compatibility with mobile clients, so Masto issue 1209 might be worth a quick glance if it gains more forward momentum.