I really don't like that idea, because it makes it takes so much of the game away from regular players.
For one, getting to and killing the pirate king should be a herculean effort. Getting to him and fighting him should be a massive undertaking in and of itself. And the more I think about it, the less I like the idea of the King being the kind of fight where you take 5-10 people against. His officers should be those sorts of fights, where the hardcore players get to take pressure off of the town by affecting the chain of command.
But no small group of players should have the means or capability to promptly end the event, or to milk the event for secret content only they get to have. If the server is going to try and down the Pirate King, it should be the kind of event the server really invests in. He should be more like a world boss, who can wipe the floor with a 40 man raid of even the best players if they aren't playing at their very best. He should be the kind of boss where everyone hears about him going down and think "DAAAAAAAAMN."
I wouldn't make him a world event in the sense of every single player being able to fight on equal footing, by incredibly buffs, like you sometimes see. Would I would do is make it a group effort with regards to the entire scenario.
-To have a chance of defeating the Pirate King, you need to take out his officers, so they can't offer him assistance. And they'll have a respawn timer, so they'll all need to be taken out within, say, 1-2 days.
-To have a chance of defeating the Pirate King, you need to cut the avenues of support from his troops, both in the waterways and on land, which means capturing and holding key locations while the King fight is happening. And to make it interesting, the King will issue different orders to try and circumvent this issue, meaning the more casual players manning those stations need to react.
-To have a chance of defeating the Pirate King, you need a big group of the best players around. The kind of players who can walk into an encounter with some randomized mechanics and abilities and potentially survive and win.
-These lesser tasks obviously can't just mean stationing one level capped player there and knowing it'll turn out well. They should level scale and be appropriately difficult for anyone involved.*
*The other side of this is that, should you get 100 people at each point, they shouldn't be difficult to hold. Because it means that the server is overwhelmingly deciding to support this town, instead of dealing with some other event going on. Essentially, they are trading worse scenarios everywhere else to get a better scenario here.
And the point is that the Pirate King is supposed to represent a massive achievement. Defeating him is like downing heroic Lich King. Because downing the Pirate King actually has an effect on the rest of the world. The point is for the good guys to never come out on top. Sometimes, they just manage to defend the village. Sometimes, the village gets razed. And sometimes they actually take off the dragon's head.
The point of this world is that everything needs to gain value because you can't do it all. Choosing to spend the effort to take down the Pirate King might mean you're sacrificing some northern villages to their new Dragon tyrant. It might mean the players of your server don't care that Centaurs are gaining ground in the southern desert.
It should be a game of survival. You pick what you care about, and you fight for it. But you don't save the world. Or at least, you only save the world when it's a massive, server-wide scenario.
I'm flat-out tired of games where you get to wrap everything up with a bow, and where hardcore players are always the ones to do it. I'm fine with hardcore players getting more to do, but it still needs to be challenging, and they can't hold the world in the palm of their hand anymore. It just isn't fun for the 95% of casual or non-capped players who will never get to be included in that stuff. And that will kill any MMO.