Allow moderators to pin posts to that site's sidebar, individually or by seasonal event
One neat feature on SE was that mods could mark posts as [featured], and any such-tagged post would appear in a special sidebar widget. That way meta posts which the per-site mods felt deserved individual attention could be pinned for people to see (often community events and site policy discussions). Ideally this would be a category setup toggle (i.e. "allow featured posts from this category? Y/N"), but even an all-category mod-only [featured] tag would be amazing.
Seasonally Featured Tags
Additionally, SE featured site calendars, wherein a mod could specify a date range for an event, and in the period leading up to and during the event, users could click on the link in the calendar to view all questions related to that event. For instance, consider an agriculture-centric site which celebrates a harvest festival every year on August 4, wherein everyone gathers to cut, collect, and separate the summer's bounty. Such a site might want to display "Harvest Festival upcoming on August 4," and clicking on that would show all questions tagged [harvest], [gathering], and/or [winnowing].