This would sound...very difficult despite the simplicity on paper to implement. But for long-scale future events (Late 2016-2017 I mean).
A sort of Invasion Event. It would go like this;
An in game announcement names a city and says the invasion begins. Rush to the portal in the city to protect the town.
Example; "Port of Winds is being invaded by Pirates!" Followed up by a portal opening in the Port of Winds. This is to avoid just putting monster spawns in every city harming afk'ers. The Portal leads to a Channel 100 type event map where gauntlets of enemies from that area will begin to invade in multiple waves. Their levels are fixed however they're meant for high-number groups so think Black Claw Nest mob hp. At the end of the invasion; One or Multiple bosses spawn based on the area (Port of Winds = Hookah, Alvida, Kalygon. Libra = Rokko, Vella, Athoo. for a few examples.) The bosses are the hard part being stronger, faster, higher hp, or even new mechanics. Once they're dead, event rewards players, specializing in increased rewards for higher damage, most healing, boss finishing blow, etc.
Rewards can be adjusted for this. I'm open to ideas on fair rewards that won't make this event open to abuse.
Just for examples of what you can put as the attackers. Names mean both the mobs and bosses/mid bosses;
Odelia: Sangka/Kungka, Vagabond, Vegas, Maybe even those Wolf Den bosses.
Port of Winds: Hookah, Alvida, Kalygon
Moonlight Shore: Artis/Night Lord, Athoo, undead bosses from Graveyard.
Libra: Rokko, Queen Vella
Kazeura: Mission bosses from Salvalon/Lamieta, Dungeon mid-bosses from Akia Cave
Fungoid: Bubobubo, Burlune, Karkharous
Ash Mist: Pythanous, Arkah, Agnis, Avice, Van Cliff
Ellora Camp: Alexander, Kryos, Drakos, Dragon Valley, Edge of World.