Anacron-like timers (e.g. "run once a week, doesn't matter when") would require persistent timestamps in order to cope with reboots and downtimes.

I imagine that would involve keeping unit-like files in /var/lib/systemd/system (or $XDG_DATA_HOME/systemd/user) saving some of the timestamps with sections like:

[Persistence]
ActiveEnterTimestamp=...
InactiveEnterTimestamp=...

So that OnUnitActiveSec and OnUnitInactiveSec will be able to fire when the unit was last activated in a past boot.

Persistence of timestamps for a unit would be enabled if any timer enabling persistence and referencing the unit is activated.

Of course, anacron supports more features like randomizing start times to prevent stampeding and limiting the time-of-day range the jobs can be run, but this would be a nice start.