MRPG The Watcher Raid
The Watcher Raid is an invasion-style raid in reverse - with the player characters acting as the invaders. The flagging quests for it tell a story of an evil monster imprisoned in a pocket dimension, called simply "The Watchers" (they may or may not be cheap ripoffs of beholders). The Watchers must be killed, for some reason told in song and story that pretty much boils down to "weird looking monsters spying on us, ewwww!". Once the flagging quests are complete, players are ready to start the raid.
The Watcher's Prison
Players may teleport in to the prison from staging areas in three cities. Twelve player characters are required to start the raid, and up to 40 can enter the instance at a given time. The Watcher battle has no phases; there will be 5 or 6 monsters with no trash mobs. The Watchers have high HP, strong debuffs, and lots of spam instant-kill attacks. When players die, their bodies are immediately transported back to the staging areas.
The idea is based around large-scale, simple coöperation. Healers waiting in the staging areas will be able to quickly raise and restore fallen players outside the raid, and characters can switch in and out as they please. The raid will not have a time limit, and may continue until the Watchers are defeated.
This is a tough raid for rewards, since participation will be very difficult to measure. The healers waiting outside won't even be counted as participating, technically speaking. And the basic concept here is to let as many players as possible get their licks in.
Perhaps the best way to complete this raid is to make it more an event, and provide server-wide rewards for completing it.
Another option is a token system. Players inside the dungeon get tokens for inflicting damage, and players outside get tokens for healing them.