As adventurer's looked to the skies they could see it growing a cloud of darkness looking to engulf the world. Hope withered and where the darkness had already consumed lay in a dormant sleep, the wilderness growing more aggravated and restless as if the darkness is strengthening, corrupting and enraging the beasts.
Divided by time, Adventurer's will need to band together to fight off this impending doom as the monsters of the dark invade our towns, destroy the peace and serenity, murder and plunder all those that have come before us.
I suck at role playing. That said:
-Monsters that scale to your level
-Global counter for each town where everyone needs to kill to make it up
-Waves of sorts that get stronger and you can reclaim the inners of cities slowly or if not defended lose them Taking away the spawn
-Boss at the end that has end rewards that stack up to your contribution
- Would be key idea to defend the airships and spawns.
-Qieros runs back to the churchs of each town
heheh 
I like the idea. The global counter thing could be fun. It would encourage travel and group play. I'm not a smart person so im not sure what you mean by monsters that scale to your level. Unless you mean the higher the level the stronger the monster. I also think that keeping strong monsters in one spot isn't a good idea. It causes friction between players being limited to where to fight. LOL
fighting slimes, then fighting imps, then progressively getting harder in the cities is a good idea except it couldn't be in the actual city it self. To many npcs to many innocents in the way. Perhaps a new village could be created to houses this on going battle.
WITH A BOSS Fight at the end!! However If that is the case, I don't believe these types of quest should be repeatable. Someone strong enough could go at it alone and take advantage of the situation.
But as I read this I was thinking PIRATE BATTLE! And that would be just freaking awesome !! lol Good story though