I think that there are multiple solutions out there which satisfy one of your use cases somehow, but not a one-fits-all solution.
At Ulm university, we have also been thinking about the issue of publishing important news (which we regularly run into), but haven't built a real solution yet.
If this is mainly about administrative announcements, there are two existing plugins from Catalyst IT on https://moodle.org/plugins/local_envbar and https://moodle.org/plugins/auth_outage which already produce these kind of banners and which could be a basis.
If this is more about announcing important news, like the fact that there are new Moodle trainings upcoming for teachers, I would be in favour of something like tool_rsstomessage which I pitched in https://moodlemootdach.org/mod/forum/discuss.php?d=86.
And, don't forget the announcement forum in Moodle core which can be used on the Moodle frontpage and which all users can automatically be subscribed to.
So, my main query would be to collect _all_ use cases first to be able to see the big picture and to be able to talk about solution approaches.