I'd kind of like if there was some ability to have more complex multi-part TTLs ...

I'd kind of like if there was some ability to have more complex multi-part TTLs as an option along with a default TTL (the current one). So I could specify

  Default TTL: 12 hours;
  [<startdate> to <enddate>] TTL: 10 minutes;

or even

  Default TTL: 12 hours;
  [Thu 0000-1200, repeating]: 5 minutes;
  [<startdate> to <enddate>] TTL: 10 minutes;

So with no further effort all downstream caches/clients can basically have advanced notice of regular maintenance windows as well as planned maintenance and just automatically adapt. Of course this wouldn't deal with true emergencies, but it might lower the overhead for a huge amount of regular stuff that otherwise tempts people to set it low and leave it that way.

I dunno, I'm sure there's other downsides I haven't thought of, and proper implementation would require thinking through side effects. But after a long time dealing with it feels like there's some room for something beyond one single TTL ever which must be specifically changed (with a wait for propagation) well ahead of time whenever anything planned with a risk of issues needs to be done. Maybe?