Actually i have two suggestions, but they correlate a bit, so i merged them into one thread.
1.Siege mode.
Let the game atnagonist (Arcane if i got the story right) send his strongest minions to vipe out Terminus. Once in a while Arcane himself may lead his army. Since these are the strongest monster there are, the event should be end-game oriented. A the beginning i thought this event should be held only at the map of Terminus city, where end-gme monsters should have their revenge, but it's quite possible to hold it at two maps - first a map near the fortress itself (but not Logging Area). Then, if the monsters weren't completely defeated, they may spawn insde Terminus. But it may be held only inside Terminus, viping out not only the players, but NPC too (put them into hospital so they won be avilable for some time - an hour or two).
2. GM-challenge.
It's a PvE-PK-PvP mixed event. Any GM whenever they want may spawn as any monster/boss of their choice (strongest monsters/powered up worldbosses preferred) either on a special premade PvP/PK map or at a map of their choice (Terminus included) and start the massacre! Players are free to join into raids/parties and use any strategy they can. GM are limited to the skils and possibilities of the monster they spawned,but use their own gaming skills to make it harder.
Haven't yet thought about the rewards of both evens, but well this is only a basic idea. If it will be accepted we (game comunity) may figure out something nice