The following are my two cents regarding one-time events.
I would prefer that ‘one-time’ events use the build phasing demonstrated in patch updates already, in combination with an account-wide tag. Basically, have one-time events repeat, but prohibit repeat completion of the event.
The event would repeat every 3-6 hours or whatever, but in different overflow instances. Players that had already fought the Ancient Karka would be directed to a post-Ancient Karka instance. Players that had not yet fought the Karka would be shuttled into an overflow where the event had not happened yet.
So, everybody gets to fight it once, preserving the important world-changing feeling associated with one-time events – but without the timezone discrimination of actual one-time events.
Couple this with making sure to reward players who, for whatever reason, got their account tagged as ‘has-fought-Karka’ without getting rewarded and I’d be a very happy customer.
For extra bonus points, link this account tag to other zones so they phase in the same way.