A special one-time event? By the way it looks and sounds all the servers must be closing for good.
Bad jokes aside, I am certainly curious and wonder if this truly will be 'one-time' especially if the event proves to be very successful.
It's been done before, although that was a decade ago - the event in question was the Devil Invasion, which saw Devil forces spawning in the Marketplace for a few weeks and ultimately saw the Marketplace Tent destroyed in a massive explosion. That event was a little different, as it was a race between servers to complete a certain number of endgame raids, whereas this event has been described as collaborative across all the servers.
The purpose of this event is to permanently unlock the U63 quests, which you can only really do once, so I don't see this particular event being repeated, but that's not to say they might try similar events in the future if this one works nicely.