IDK what they’re thinking, however I would imagine something like TRP where everyone with the addon can see a specific RP calendar (whether tied into the current in-game calendar or not, an entirely different story) and create events to share.
An RSVP system would be great, the ability to filter or sort events by faction, time, popularity, type, etc. On top of RSVPs, also reminders that the events are coming up at week, 3 day, 1 day, 6 hours, 3 hours, 2 hours, 1 hour, 30 min, 15 min increments that individuals can of course turn off and on, or even custom setup.
I would add some limits to such an addon to prevent bad actors from being themselves. Can only add X amount of events in a Y period of time, ideally per account but may have to be per-character. Events can only be deleted by their creator. Events can only be scheduled out so much time in advance. Events can overlap so not to exclude anyone.
Controversial one and perhaps can be refined or figured out over time, but a system to create calendar ‘admins’ or ‘moderators’ that can remove the inevitible spam and troll events. Its difficult to give everyone access to a calendar, even with the limitations above, and not expect someone to spam 7 events with 7 characters every 12 hours, or whatever, that need cleaning up and potentially banned from creating events. How possible that is is anyone’s guess, I assume it wouldnt be, but having some sort of moderation would be needed if its going to be used long term.